I know that the movies were written to be a stand alone universe on their own, but what key elements were missing in the movies that were vital in the books that viewers are missing out on?
Here’s an incomplete list of the biggest things I can remember that were omitted or simplified significantly in the movies versus the books. The whole post is riddled with spoilers so I won’t even try masking them; you have been warned:
Probably the largest single overt omission from the entire movie series was the character of Peeves the Poltergeist. He plays a relatively minor role in most of the books, but the comic relief would have been good in areas, and his role became more important in book 5 and book 7 as he actively fought against the bad guys.
The movies in general leave out a LOT of Quidditch matches from the books.
The existence of Squibs (non-magic children of wizarding parents; the reverse of a Muggle-born) is never explicitly stated, though the characters of Filch and Mrs Figg (both Squibs) are present.
The Sorcerer’s Stone movie is probably the closest overall to the book, as the book is the shortest.
- They miss an entire game of Quidditch against Hufflepuff; it’s notable because Snape referees the game, piling suspicion on him, and thus Harry’s under pressure to end the game quickly. He catches the Snitch in less than five minutes, which has to be some kind of record.
- The film simplified how Harry and Hermione get caught out of bed at night, meaning Neville isn’t out with them in the Forbidden Forest (simplifying that scene).
- The characters of Bane and Magorian are omitted. In the book, a couple of conversations introduce the centaurs as star-gazers and a very proud race.
- Malfoy doesn’t challenge Harry to a duel in the movie; this simplifies their discovery of what’s behind the door on the third floor.
- They skip two of the puzzles in the dungeons (one of which takes about a paragraph in the books; another troll, which is sound asleep when they get there), and simplify how they deal with Fluffy (there’s a harp that’s already playing magically).
- In the book, the final fight in the mirror chamber ends with Harry blacking out from the pain in his scar, locked in a death grip with Quirrell just as Dumbledore comes upon the scene. We’re told that Quirrell died as part of the exposition in the infirmary, as well as a few other things, including some tantalizing missing information about why Voldemort would want to kill Harry in the first place.
The second movie was shot pretty faithfully. A few things are simplified and a couple things are switched around, but I can’t remember much that was overtly omitted other than:
The side trip to Filch’s office early in the book, in which the audience learns about Squibs and that Filch is one, never happens.
Sir Nicholas’s 500th deathday party on Halloween night was omitted; in the book, HH&R’s attendance of this party leads to them finding Mrs. Norris, petrified and hanging by her tail, at just the wrong time. The movie uses a combination of Harry’s detention with Gilderoy Lockhart and a chance meeting with Hermione and Ron to accomplish the same effect.
Probably got the biggest cuts in regard to Quidditch when going to movie form; in the book, the Quidditch matches are very important plot points concerning Harry’s weakness with dementors and his learning of the Patronus Charm.
In the book, Harry and the Gryffindor team overcome Harry’s first-ever honest loss in Quidditch during the match against Hufflepuff and Cedric Diggory (in which he fell off his broom due to the dementors, the one scene the movie does show), to win the Quidditch Cup for the first time in ten years by beating Slytherin.
Harry also is able to use his new Firebolt in that match against Slytherin, after having it confiscated by McGonagall when he first got it under suspicion of it being sent by Sirius Black (still thought to be Harry’s mortal enemy). The movie ends with him taking it for his first spin, with no question of who sent it or their intentions.
The replacement of the Fat Lady with Sir Cadogan after Black’s first introsion into the castle was omitted; this also omits the incident where Sirius actually gets into the Gryffindor dormitory.
The exposition of Sirius as Harry’s godfather, and that he apparently betrayed his parents to Voldemort, was drastically simplified. Also, the snowball fight was actually a mudball fight, and Harry’s cloak comes askew so Malfoy sees his head floating in midair, meaning Harry has to dash back to Hogwarts to avoid being caught.
Harry takes many more lessons to properly learn to cast the Patronus Charm.
The meeting in the Shrieking Shack was cut down a bit, and there were some other inconsistencies (this movie actually added more than it removed in the way of small details, if I remember right, in order to add some time and comic relief on events that play out much more quickly in the books, like getting past the Whomping Willow).
The fact that Lupin, Pettigrew, Sirius and James (Mssr’s Moony, Wormtail, Padfoot and Prongs) were childhood friends is implied throughout the movies but they never went into much detail; the third book has a lot of exposition about these four.
Exactly who wrote the Marauders’ Map is implied by Lupin’s familiarity with it, but it’s explained more explicitly in the book.
Harry’s leaving the Dursleys’ to go to the World Cup is omitted; in the book, the Weasleys try to connect Harry’s house to the Floo network, but the Dursleys have an electric fireplace insert that blocks their path, and Arthur ends up destroying half the Dursleys’ living room in the process of extricating them.
The fact that Cedric beat Harry in their Quidditch match last year (because Harry fell off his broom under the effects of the dementors) was omitted because it wasn’t mentioned in movie 3 (it’s implied, not very well, that the match was stopped after Harry fell). This gets Cedric and Harry off to a rocky start in the books.
Ludo Bagman is just a big red herring in the books and is absent from the films. Because he’s not there, the Weasley twins take over the undertone of gambling on the Triwizard Tournament.
We get scenes from the World Cup, but we never actually get to see the game being played. This is in keeping with the general reduction of the role of Quidditch in the films compared to the books.
Harry and the Weasleys are in the Minister’s box as well, alongside the Malfoys, instead of in the upper deck.
Dobby’s role is cut out completely (the hint to use gillyweed instead played out the way Moody had intended it to in the book, with Neville telling him after being given a book on Herbology by Moody), as is the side story about S.P.E.W. and Winky which feeds into a much more complex story concerning Barty Crouch and his son.
Pretty much all the challenges play out slightly differently, but the spirit is there.
Charlie Weasley’s character is omitted as an acted part; he’s only referred to in film 1, but he has a bit part in both books 1 and 4.
Rita Skeeter’s poking around and truth-distortion is hinted at but much reduced; the fact that she’s an Animagus and used her talent to literally be a fly on the wall was completely ignored.
The dragon Harry faces in the book is not chained, and is much less aggressive, but stays rooted on her nest most of the time because she’s a nesting mother (which is the challenge for the contestants, to get the egg away from a very defensive dragon).
The judges’ ratings are pretty much ignored until the last challenge, and that last challenge is very different (the original maze is much more of a true obstacle course, with a lot of magical beasts and other tricks).
The Blast-Ended Skrewts are completely absent because they’re no longer needed for the last challenge.
Cedric and Harry are much more sportsmanlike to each other in the book; the point of the change of the maze itself in the film was to sorely test that because the movie didn’t have the initial enmity between these characters over their last Quidditch game.
In the book, Harry saves Cedric, but is injured and unable to make it to the cup; Cedric is faced with the choice Harry had in the film, of taking the Triwizard Cup for himself or helping Harry.
There’s more Pensieve scenes in the book than the movie; it’s condensed in the film to one scene that tells you everything you need to know, in the process removing Ludo’s backstory and the original introduction to the audience of the character of Bellatrix Lestrange.
Fudge’s denial and blatant coverup of the events in the maze (including Barty Jr’s ultimate fate) are omitted; the essentials are instead given to the audience in film 5.
This book was trimmed right down to the barest elements; I think it’s actually the longest in the entire series, but they condense it to just 2 and a half hours of movie. As such, there’s a LOT missing:
The events leading up to Harry coming to Grimmauld Place are condensed a bit, especially the confrontation with the Dursleys over what happened to Dudley.
Tonks’ being a Metamorphmagus is hinted at but never fully explained or made much use of other than comic relief.
The cleaning of the Black house (including the later-important detail of the locket being binned) was cut out completely. It is also during this chapter, I believe, where we’re originally given the name Regulus Black; Sirius’s younger brother and a key “non-character” in later books. The movies give the first mention of this name to Slughorn as a throwaway line in HBP.
Percy’s estrangement from the Weasleys from the book is largely ignored in the film; he’s the one holding Harry in Dumbledore’s office when Ms. Umbridge confronts Dumbledore about the DA, and thus he’s an enemy, but besides a sideways glance by Harry at his captor, that fact is never overtly brought to the audience’s attention.
The assignment of prefects in the fifth year is omitted, along with the fact that Ron was chosen for Gryffindor over Harry for that honor.
Quidditch is completely absent from the film; it’s implied that the disbanding of school activity groups includes a blanket ban on Quidditch. In the book, Harry’s banned from playing after the first game, in which Malfoy’s taunting gets to him and the Weasley twins, provoking a fight. Ron first gets the Keeper job in Book 5 instead of movie 6, but isn’t very good at it (prompting a Slytherin song: “Weasley Is Our King”).
Harry’s crush for Cho is ended in a different way in the book, after a disastrous date. Cho herself doesn’t betray Dumbledore’s Army as in the movie (however unintentional on Cho’s part), but she does call Hermione out for using such a terrible curse against Cho’s friend (who DID betray them), which pretty much ends their relationship when Harry comes to Hermione’s defense.
Rita Skeeter is absent in this film; in the book, she writes a true account, exactly as Harry tells it, of the night in the graveyard at the end of book 4, which is published in the Quibbler and gets Harry in a LOT of trouble with Umbridge.
The Weasleys cause a lot more chaos in the book before they leave Hogwarts, and it’s not known at first that they’re doing it (at least not to Umbridge and the Inquisitorial Squad), until they’re caught turning a corridor into a bog.
Harry’s “guidance counselor” meeting with McGonagall, which Umbridge sits in on and in which Harry states his desire to be an Auror, is omitted completely from the film. This tidbit is first given in movie 6, more or less true to that book.
In the book, Harry, Hermione and Ron visit St Mungo’s to see Arthur Weasley after the snake attack, in the process running into their old DADA teacher from book 2, Gilderoy Lockhart, who is a long-term patient there after the backfired Memory Charm. The fact Arthur was at St Mungo’s at all is glossed over in the movie.
Harry’s spending the Christmas holidays at Grimmauld Place is trimmed; Snape meets him there to tell him about his upcoming Occlumency lessons. In the film, St Mungo’s is skipped entirely, and so the Occlumency lessons commence immediately once it’s clear Harry is seeing into Voldemort’s mind.
Firenze’s part in Book 5 is completely omitted from the film, as is the subplot of Professor Trelawney having a taste for cooking sherry leading up to her being sacked.
The centaurs were portrayed much more simply in the movie, and the fact that one centaur, Bane, wanted to kill Harry and Hermione as well (for having the centaurs do their dirty work) was omitted.
Dumbledore projects a little more outward emotion over what Umbridge is doing to his school in the film than the book, and seems less confident in her presence.
Harry has a lot more lessons with Snape than are shown, and is kicked out of them because he purposely looked at Snape’s memories in the Pensieve, not because he saw them by accident using the Shield Charm to rebound the Legilimens Charm (though that does happen).
The OWL testing is hinted at in the movies, but there’s much more in the books; all the practical spellcasting including another Patronus shot right at Umbridge, Harry fainting and seeing Sirius tortured during his History of Magic test, seeing Hagrid attacked during Astronomy; all omitted.
The Department of Mysteries is much simpler in the movie, mainly centering around the Hall of Prophecy and the Ampitheatre. All the little oddities in some of the other rooms in the book, and the role they play in the fight, are omitted. The fight itself is slightly truncated as a result; there were more of the kids still able to fight when the Order shows up in the movie than the book.
The fight between Voldemort and Dumbledore is changed; the spirit is there but the spells and other events are perhaps more visually impacting.
Dumbledore’s explanation to Harry after the events in the Ministry is cut WAY down. In the book, that’s an entire long chapter basically explaining Dumbledore’s behavior from the very first book up to the present, including why Voldemort wanted to kill a one-year-old Harry in the first place, and which also features a full-out raging teenage meltdown on Harry’s part.
The movie was closer in spirit than 3, 4 or 5, but as the second-longest book (and the one with the most exposition) it was cut considerably as well:
The opening chapters of Book 6 give you the same information presented in the movie, but in a very different format involving the Muggle Prime Minister.
Bill and Fleur’s courtship, and the angst that causes the Weasleys, especially Molly, is absent from the film. We’re not even told about Bill in the films until DH part 1.
The subtext of Harry’s Auror ambitions is present but diminished in the film. We don’t see Harry’s OWL grades, but we’re told the basic information we need to know about why he hadn’t signed up for Potions, and thus how the HBP’s Potions book comes into his hands.
Snape is now the DADA teacher, and the sixth years are expected to begin mastering nonverbal spells. These facts are skipped in the film.
The book had a lot more memories that Dumbledore had collected, relating to what Tom Riddle was doing in his late years at Hogwarts and immediately after leaving school.
Levicorpus is just another spell, not one of the HBP’s inventions. In fact, it was actually first used in film 5 (Luna uses it on a Death Eater at the Ministry), when it isn’t introduced to readers by name until book 6 when Harry uses it accidentally on Ron.
The fact that Katie Bell was a Chaser for Gryffindor gets one throwaway mention in the movie, and the fact that her being out for several weeks hindered Harry’s Quidditch team was completely ignored. The character of Katie as portrayed in the film was much younger; she should be a seventh-year by this point, the last remaining member of the team from books 1-3, but the casting made her look more like a fifth- or sixth-year.
Cormac McLaggen had a bigger role in the books, primarily in the omitted second Gryffindor game of the season, where McLaggen is such a bad team player and such a know-it-all about Quidditch that he ends up taking Harry out with a bludger in the second game of the season.
In the book, Felix Felicis doesn’t make you act as drunk or loopy as Harry was in the movie, and Slughorn isn’t stealing Tentacula leaves when Harry meets him.
Apparition lessons with Wilkie Twycross from the Ministry are omitted, though Apparition itself is present at key points in the story.
In the book, whether Ron is going to live or die after being poisoned is much more in doubt; in the movie, he sits right up and cracks a joke. The next scene, where Ron says Hermione’s name in his sleep causing Lavender to break up with him, is not true to the book, though she is offended that nobody thought to tell what had happened to Ron, and Ron begins feigning sleep when she later comes to see him in hospital.
Rufus Scrimgeour visits Harry at the Burrow over Christmas, with Percy Weasley in tow, to try to recruit him as a poster boy for the Ministry (much like Fudge did) and to try to learn from Harry where Dumbledore keeps disappearing off to; the readers learn that Umbridge still works for the Ministry, and that not much has changed in the world of magical politics after Fudge’s ouster.
After the incident in the prefect’s bathroom with Sectumsempra, Harry has to hide the book in a hurry, and he does it himself by going to the Room of Requirement, finding a place to stash it and then decorating a nearby bust with a wig and tiara to mark it. That tiara later becomes important.
Snape sentences Harry to detention every Saturday for the rest of the year after the Sectumsempra incident. Because of this, he can’t play the last game of the season, and has precious little time with Ginny. This omission is part of a general slowdown of the blooming relationship between Harry and Ginny.
Harry’s feelings for Ginny are more manifest in the book, but shown in different ways. Because the celebration of Gryffindor winning the Quidditch Cup isn’t shown in the movie, Harry can’t kiss Ginny in front of Ron (and the rest of Gryffindor House) in the middle of it, which would have been funny to see.
The return of Harry and Dumbledore to Hogwarts after their field trip is simplified. In the book, they Apparate to Hogsmeade (because in the books not even Dumbledore can Apparate within the Hogwarts grounds), and Rosmerta, the bartender at the Three Broomsticks (who is under the Imperius Curse via Malfoy) urges them to get to Hogwarts as fast as possible using some brooms she gives them, leading to them arriving at the Astronomy Tower.
The fact that Snape was Harry’s Occlumency teacher, and that Harry was so poor at it, was ignored in the confrontation between Harry and Snape at the end of the film. The fact that Snape is the Half-Blood Prince is divulged here much as it is in the book. Harry and Hagrid putting out Hagrid’s house with their wands after it was set on fire was skipped, as was the exposition about Snape’s witch mother, Eileen Snape nee Prince.
Dumbledore’s funeral is omitted from the movie. Because of this, and the fact that in the movies Ginny and Harry aren’t publicly an item, Harry formally breaking up with Ginny (because he knows he’s not coming back to school and that Ginny can’t come with them) is also skipped; instead this is where the audience learns of Ron’s tacit approval of Harry and Ginny being together.
… was pretty close on screen, compared to the last few, but that’s because they had two movies - over 4 and a half hours - to tell the story. The main things missed were:
Ginny kissing Harry at the Burrow is a bigger twist; they’ve officially broken up, but obviously not for any love lost between them. Ron breaks it up instead of George, chastises Harry for leading her on, and takes steps to make sure they’re not alone together again until the wedding party is crashed.
In the book, Scrimgeour interrupts Harry’s birthday party at the Burrow (which itself was skipped in the film), first to try to get information from Harry about Dumbledore’s death and the Order, and second, to read Dumbledore’s will (which is seen in DH Part 1). Scrimgeour in the film has a little less animosity toward HH&R than is stated in the books, but he is still rather short with them; it’s taken as part of his character.
Harry is himself in the film during the wedding; in the book, just to be safe, he’s given doses of Polyjuice Potion to look like a redheaded boy from the nearby village, and is passed off as yet another Weasley cousin.
HH&R Apparate to Tottenham Court Road in the book, but it’s given as Shaftesbury Avenue in the film, with Hermione saying she used to go to the theater with her parents. Road names in London are a bit odd, due to the age and history of the city, and the road now systematically known as A400 includes parts of both historic streets. This isn’t a major crime, as the establishing shot is the very famous Picadilly Circus where Shaftesbury starts, which is a few blocks southwest of the major theaters on Shaftesbury. Tottenham Court Road proper is only a few blocks north of there following Charing Cross (also part of A400).
The discussion with Elphias and Ron’s great aunt is longer. In fact, the entire tarnishing of Dumbledore’s shining reputation and the introduction of his backstory was a subtext almost completely removed from the films, with only the barest elements given to the audience where it’s absolutely essential. This results in a reduction in Aberforth’s part late in the second film, and of Dumbledore’s at “King’s Cross”.
Viktor Krum is in attendance at the wedding in the book, having been invited by Fleur. Harry makes the leap that Gregorovitch is the wandmaker that Voldemort is looking for after remembering that Krum’s wand was made by Gregorovitch (from WAY back in Book 4 during the weighing of the wands).
Harry, Hermione and Ron spend more time at Grimmauld Place planning the raid on the Ministry, and Kreacher actually warms to them after the confrontation in the kitchen over the locket and Mundungus Fletcher, before HH&R are forced never to return after escaping the Ministry.
Tonks and Lupin being engaged/married is brushed over in the films. In the book, it’s a much bigger subtext; Lupin shows up at Grimmauld Place while HH&R are planning their infiltration of the Ministry, and we learn he wants to leave Tonks because she’s pregnant, and he’s afraid what the child will be (he’s a werewolf after all). Then, while HH&R recuperate at Shell Cottage later, Lupin stops by and gives the news that it’s a boy, and asks Harry to be godfather. All of this was removed from the film plot, which I think was a mistake; it removes some of the power of the events of the fight.
Phineas Nigellus is absent from the film (and pretty much all others; he has a bit part in book 5 as well). This forces the film to be very vague about how Snape could have known where they were to get the sword to them; there are memory images in the Pensieve scene but no details.
In the book, the spark for Ron leaving Harry and Hermione is an omitted conversation between Tonks’ father and two ex-Gringotts goblins, overheard by Harry, Ron and Hermione while camping, regarding what’s been happening in the outside world (particularly to Ginny, the last Weasley at Hogwarts).
Harry had to make a conscious decision in the book, choosing Horcruxes over Hallows, by choosing to stay at Shell Cottage to get the information he needed from Griphook and Ollivander, instead of acting on the knowledge he has through his connection to Voldemort regarding where the Elder Wand is and that Voldemort also knows. In the film, there was never a chance that Harry could have gotten to the Elder Wand first; HH&R were still recovering from shock and injury from the events at Malfoy Manor.
At Gringott’s, Harry uses a few more Imperio charms in the book, and also Confunds a few guards at the door.
In the Lestrange vault at Gringott’s in the book, the treasure has not only been guarded with Geminio, but also the Flagrante curse, which causes the copies to be burning hot to the touch, making HH&R’s entrapment in the burgeoning treasure that much more painful.
Griphook’s role in the movie is close to the book; in the book we’re not told he’s killed at Gringott’s, which I think is a good foreshadowing, as Neville getting the Gryffindor sword from the Sorting Hat at the very end could be confusing to a reader who knows it was last seen in Griphook’s hands.
The Carrows are omitted from the movies as speaking roles, through they’re mentioned in the right place in the film and it’s inferred the two of them are the ones flanking Snape in the Great Hall; in the books they’ve taken over Muggle Studies and the class formerly known as Defense Against the Dark Arts, and Harry uses Crucio on one of them in Ravenclaw Tower to protect McGonagall (for the first time seeing what Bellatrix meant: “you have to mean an Unforgivable Curse”). McGonagall also uses an Unforgivable (Imperio) on the other Carrow in the same scene of the book, showing that Harry’s uses of them in several situations, while extreme, are not unique among the good guys and so not necessarily “unforgivable”.
In the book, after the Grey Lady (Helena Ravenclaw) confirms that Tom found her mother’s diadem and brought it back to Hogwarts as a Horcrux, Harry remembers putting a tiara on the bust he used to mark the location of the Half-Blood Prince’s potions book; that’s what allows him to find the tiara again. The film simplifies this by inferring Harry is attuned to the Horcruxes and can sense their presence.
Most of the battle plays out slightly differently; the whole scene with Neville on the bridge isn’t in the book, and there aren’t as many of the past characters shown in the movie as the book has. Colin, Wood, Lavender, Cho, Percy, Bill, Charlie, Alicia Spinnett, Katie Bell, even Grawp and Kreacher are all mentioned, and Percy gets a reconciliation with his parents before the fight. Different people are in different places, and different things happen, but the spirit is there. I did want to see Professor McGonagall directing a herd of animated classroom desks down the hall with a most un-McGonagall-esque “CHAAAAARGE!”; her use of Transfiguration in the film was limited to one spell animating the suits of armor (more like statues in the film).
More Pensieve memories in the book than the film, of course, though they showed more than I thought they would in the movie. The film simplified how Petunia became so virulently anti-magic, why Lily falls away from Snape at school, and how Harry got the Sword of Gryffindor. There’s also a minor plot point in the book where we find out who ransacked Grimmauld Place (besides Mundungus) and why half of a letter from Lily to Sirius was missing, as well as half of a picture.
The scene at “King’s Cross” in Harry’s mind is truncated, mostly omitting the parts where Dumbledore fills in the remaining blank spots of his backstory; this entire subtext was largely absent from both parts of DH on screen.
The killing of Nagini is much more straightforward in the book; Neville breaks a “Full Body-Bind” curse Voldemort puts on him, pulls the Sorting Hat off his head (Voldemort put the hat on him and set it ablaze while Neville was paralyzed), draws the sword from it and chops the snake’s head off before the battle even starts up again. The film draws it out to a last-minute thing.
In the book, the centaurs and other creatures from the Hogwarts grounds join in once the battle starts up again. Their role is omitted, though we do see the spiders attacking earlier in the battle, having been driven out of the Forbidden Forest by Voldemort’s forces.
In the film, the audience learns why the last standoff between Voldemort and Harry plays out the way it does after the fact, instead of while the two are staring each other down as in the book (though Harry hints to Voldemort that he knows why the Elder Wand won’t work for him). I actually think that works better for the movie, but in the book, the slow-burn realization that Voldemort has lost this final duel before it even starts is a nice touch.
In the book, Harry is able to use the Elder Wand to repair his old holly-and-phoenix-feather wand (which was snapped in Part 1). In the movie, it’s implied he simply continues to use the one he stole from Draco.
Instead of snapping the Elder Wand outright, in the book Harry simply chooses to place it back with Dumbledore in the tomb and let nature take its course. If Harry dies a natural death, the wand’s power will be broken. Harry’s rejection of the Elder Wand’s power is more forceful in the film, and also more in keeping with his ambitions to become an Auror (a job which, as is illustrated many times, doesn’t lend itself to a peaceful death in bed at a hundred and twenty years old).
The “Nineteen Years Later” epilogue is slightly trimmed; Ron, who’s married Hermione, talks about passing a Muggle driving test, almost without having to Confund the tester. Harry and Ginny, who have three children, are informed rather unceremoniously that Tonks and Lupin’s son Teddy apparently fancies Bill and Fleur’s daughter Victiore, which is a little too close for comfort for Harry’s eldest son James, seeing as Teddy is Harry’s godson and so a regular at the Potters’, while Victoire is a blood relative (James’s cousin). And Draco, who married as well (though we’re not told whom in the book), has a son Scorpius, who is Albus’s age.
Everyone seems to overlook this: there is also the messed up plot-line from the 6th book in the Room of Requirement.
In the book, Harry hides the Half-blood Prince's potions book in a cupboard, and places the bust of a warlock on top of it, then adorns it with a wig and a tiara. In the 7th book, Harry remembers this and realizes that the tiara is one of the Horcruxes.
However, in the film, it shows that Ginny hides the diary, thus messing the whole thing up.
Kevin is correct, there are a lot of things. I'll point out a couple I think were missed the most. I think the biggest was that you got a nice reconciliation between Harry and Dudley in Book 7 that the movie dropped. It was a brief moment, but a satisfying one. It didn't undo all the pain the Dursley's caused, but it sort of combined with Petunia's late-series reluctant motherlyness to make them not complete caricatures.
Both Cho Chang and Cedric Diggory suffered in the movies relative to their book counterparts. They both show up in story The Prisoner of Azkaban. Harry is already "noticing" Cho (the Ravensclaw seeker). Cedric is actually the seeker when Hufflepuff upset Gryffindor due to Harry's Dementor accident. This is carried over into book 4 to enhance Harry and Cedric's rivalry (such as it is). The gang run into Cedric and Mr Diggory at the world cup camp grounds, and Mr Diggory is sort of rubbing the Quidditch victory in Harry's face (indirectly; he's trying to trumpet his son's win). It's a series of character moments. The Cho story line in the book sort of putters out but I still missed her getting introduced in Azkaban the movie.
Watching DH last night, I had to explain to my husband, who hasn't read the books, what splinching was. Because they didn't include Harry, Ron and Hermione learning to apparate, the audience doesn't learn what splinching is. I found that when they got to HBP and DH they took for granted that most had read the books. Not explaining where Dumbledore got the ring, why Voldemort chose to make something from each original founder a horcrux (they don't even mention that he did this in the movies, they don't tell you that the cup belonged to Helga Hufflepuff), where the piece of glass came from and why it's so important, why Kreacher obeys Harry etc… That is what bothered me in the last few movies. I feel bad for those that didn't read the books before seeing the movies...
The visit to St Mungos in OoTP is mentioned, as is the meeting with Lockhart, but what I'd consider one of the most important parts of that chapter is not. The fact they see Neville, his grandmother, and crucially, his parents and the state they're now in. It really emphasizes what Neville is personally fighting for.
Also, the big plot point of Snape overhearing Trelawney's prophecy and Dumbledore telling Harry that it could have meant Neville rather than himself.
And there was no scene with Sirius giving Harry his half of the communicating mirrors, with Harry pulling out a random shard of glass at the start of Deathly Hallows and seeing a random eye in it (no mention as well that he thought that eye was Albus's, not Aberforth's).
A few more details:
In The Goblet of Fire after the dark mark is cast during the celebration of the quidditch match, the whole scene with Winky having Harry's wand, and the ministry viewing the wands last spell seeing it as the dark mark was removed.
In The Order of the Phoenix when Harry, Ron, and Hermione break into Umbridge's office, Harry actually uses her fire to call Grimwauld Place, like Sirius does to talk to Harry, to see if Sirius is there or if his vision was true. Not to attempt to use the floo network. This is when Umbridge grabs Harry's head and pulls him out of the fire.
In The Half Blood Prince when Harry and Dumbledore return to Hogwarts, before Dumbledore is confronted by Draco and the other Death Eaters, Dumbledore actually uses Perfectus Totalus (the body binding curse) on Harry preventing him from interfering with Draco's task. Making much more sense then Snape telling him to be quiet.
Also in the sixth movie the story behind the horcrux ring was totally omitted, making it seem as if it were just some ring and not one of Voldemort's family heirlooms. (I think this is right but I can't be sure as it has been a little while since reading the book)
HP and the Chamber of Secrets movie is the most on-book with the least omissions. In my opinion, the movie is only missing one key plot point that could've taken only seconds to insert: how does Ginny know that Harry recovered the diary she threw away? All the movie had to do was make Harry have the diary at the breakfast table or something. We see that Harry's room is ransacked and the diary is missing, but the movie never gives Ginny the opportunity to see that Harry had it in his possession. In the book, Gilderoy does a huge Valentine's Day celebration including little dwarfs that go around delivering cards and singing love songs. Ginny sends Harry a singing valentine, but he's embarrassed in front of everyone and in his attempt to flee, his school bag rips and the contents spill in the hallway. Ginny looks terrified when this happens, and readers are supposed to think that she is upset that Harry rejected her valentine when she really fears the diary's return.
In the magic games Dobby is the one giving Harry the Gillyweed to help him breath underwater the last moment and not Neville. Dobby plays major part in many ways that did not show in the movies that makes some scenes not completely understandable unless you read the books. Books are much more explanatory than movies if someone does not read the books he cannot understand a hell of lot of things. The last movie was catastrophic, many things were missing and many were added :(
Just to add a few more discrepancies between the novels & the movies, I would like to talk about
The Deathly Hallows Part 2.
Those who haven't read the book but went in to see the movie would be mighty pleased with what they saw. It is the most visually extravagant of the 8 movies. But I was most disappointed with it.
The reason is that the movie ignored a few side stories from the book, none more significant than that of Albus Dumbledore.
In the book, Dumbledore's story starts from Bill & Fleur's wedding & goes through till the end when Harry talks to Dumbledore in the memory. A lot of details are revealed about his childhood, teenage & his early life with Grindelwald & Aberforth.
The movie doesn't do justice at all. In the book, the tone ranges from white-as-a-dove to black-as-a-shadow with varying shades of grey in between. After a certain point, Harry & even I as a reader wonders whether Dumbledore was a saint after all.
The book takes him at the very edge of the cliff but Alas! redeems him at the climax to leave the reader (and unfortunately, not the viewer) that Dumbledore was always flesh & blood. He was as vulnerable & corruptible as any other wizard. Just a step away from being the person with an ambition of power to the one with hunger for power. He chose wisely.
I missed those doubts & disappointments creeping in the mind when each layer is peeled off from Dumbledore's face. I wont go into specifics but read the damn thing.
To redeem the movies, I would like to say one good thing about Deathly Hallows Part 1.
I really liked the way Hermione's character has been portrayed in the movie. In the book, she is her usual self playing 2nd fiddle to Harry in crucial moments. But in the movie, she is portrayed as playing the most crucial part in the scheme of things.
Her opening scene (wiping herself from her parents' memories), the heart-felt dance between her & Harry, that scene in the dead of the night, in the forest, when she stares into the eyes of Dark Thieves (I forgot what they were called) under the protection of invisibility charm. Magic!
Not to mention the first battle at Hogwarts, after Draco lets the Deatheaters in they go on a rampage in the castle when a battle ensues. Bill is also injured by Grayback, which is showed in the movie but in the book Bill and Harry already know each other and fought next to each other.
The fire at the burrow was also pure creative liberty, never happened in the book.
Also in the 6th movie we dont learn that Sirius left his family home to Harry and that Harry became Kreachers master. In the book, Harry also sent Kreacher to Hogwarts and had him and Dobby follow Draco to find out what he was up too.
We also learn about Lupin and Tonks in book 6. They are not all of a sudden married like it shows in the movies.
Tonks is the one who finds Harry on the Hogwarts express when Draco puts the spell on him and the invisibility cloak, not luna as in the movies.
Also in the book luna is much more present than it portrays in the movies.
Harry also knew more about the Horcruxs than he let on in the movie. Dumbledore showed him the memory that showed they might have something to do with Ravenclaw and Hufflepuffs cup
They also left out the part in DH, in Godric's Hollow, where people from the wizarding world have written encouraging messages to Harry. Though it does show the house and the sign, this part was left out. I though that was a really touching part of the book and also gave Harry the strength to finish what he needed to do. The goodbye with the Dursleys was another thing I thought they could have taken the time to include.
Also in OOTP, they omitted the trip to St. Mungos. They therefore had to change how we learned about Neville's parents.
Also in Harry Potter 6 in the book they have a meeting on the train with Professor Slughorn and the rest of the "Slug Club". In the movie they postponed it till later. Plus Slughorn keeps inviting Harry and Hermione to more slug club meetings ( Harry is always busy) and Ron gets really angry at them because he is being excluded.
I am not really sure which book this was in, but they never mentioned why every year they change the DADA's teacher. For those who does not know it is because Tom Riddle /Voldemort/ was after this position. When he couldn't get it, he cursed it that no one will teach this subject for more than a year.