Why didn't Dobby give Harry the gillyweed in the Movie?

Question

In the 4th Harry Potter book, Harry Potter and the Goblet of Fire, Dobby gives Harry gillyweed right before the second task, while in the movie Neville gives Harry the gillyweed. Why did the movie producer/director make this change?

Answer 1

I want to add something to the fez answer:

Actually the decision of making Neville to give Harry the gillyweed is not random, it is somewhat inspired by the books. I read somewhere that the decision to take Dobby out was the high economical cost of making him appear virtually for the purpose of one non-critical scene. Also we should remember that the producers make the film helped by JK Rowling, who was likely the person that gave them the idea to replace Dobby with Neville to make the film closer to the book.

Let's dive into the books about the decision why Dobby was replaced by Neville:

Barty Crouch Jr, disguised as Alastor Moody by the polyjuice potion the entire academic year, put the name of Harry on the Goblet of Fire in order to make him take part of the competition and, on the third task, carry him to Voldemort to take his blood and make Voldemort alive again.

However, in order to achieve that, Harry must stay alive until the end of the third task. On the first task Crouch Jr convinced Hagrid to show the dragons to Harry and, after that, the DADA's professor helped Harry to overcome the task using the broom.

After that, on a DADA class, Crouch Jr showed the students the unforgivable curses. Neville was in panic when the professor used Cruciatus because that was the spell used by Bellatrix Lestrange, Lestrange's husband, husband's brother and Crouch Jr to torture Neville's parents almost to death. Neville in panic is even shown on the films. After that class, Crouch Jr. as an apology to make Neville suffer, gave him a botanical book because he was good in Herbology. However, Crouch Jr objective was that Harry asked Neville how to survive underwater and Neville would advise him using gillyweed because Neville would have learned that while reading the book. However, Harry never asked Neville, so, Barty Crouch Jr ordered Dobby, which he knew that was Harry's friend, to do something close to the professor while the professor was talking about the second task with Mc Gonagall and indirectly letting the elf hear about the gillyweed and come to help Harry, because this fact, Dobby in the book gave Harry the gillyweed.

Source: Barty Crouch Jr tells the written above in the end of the book to Harry and Dumbledore after giving him Veritaserum to make him to only say the truth.

Answer 2

Neville gave Harry the Gillyweed because Dobby was not in the movie because many characters and scenes were cut and/or merged into smaller scenes.

Goblet of Fire is almost twice a large as The Prisoner of Azkaban, so it was almost impossible to translate the entire book from page to film. A ton of characters and scenes were cut or amalgamated into once scene instead of several (the Death Eater trials and Harry's conversations with Sirius being a few examples).

From the behind the scenes extra on the Goblet of Fire Ultimate Editions Blu-ray (which can be found here) at 18:18 the director, Mike Newell, when talking about Brendan Gleeson, says:

He'd say "There's a great bit in the book". And you'll think, "Yeah, yeah, Brendan. Okay, fine, I know about the book. You know the book is 750 pages, and I've got 120. What do you want?" But you'd look at it, and you'd think, "No, actually, maybe he's right there"

If he really only had 120 pages of script then that's 16% of the original book they had to work with, which means a heck of a lot of stuff had to be cut.

In the same behind the scenes video (starting at 5:21) the following is said by the narrator and Mike Newell:

Narrator: At over 730 pages The Goblet of Fire was J.K. Rowling's longest book to date. Big enough for two feature films. The film makers narrowed the focus of their narrative.

Mike Newell: There was one side of the book which you could string the pearls on which was the thriller side of the story.

I'm unsure of what the phrase "string the pearls on" means exactly, but it seems as if they had decided to only take this "thriller" part of the book and make that the main focus. Meaning that anything (and anyone) who wasn't core to this telling of the story would be removed.

Wikipedia also takes note of the changes and has two quotes from Mike Newell regarding the changes to the film taken from Harry Potter: Behind the Magic (2005):

With the Goblet of Fire novel almost twice the length of Prisoner of Azkaban, the writers and producers reduced certain scenes and concepts to make the transition from page to screen. Director Mike Newell described the problem as one of "compressing a huge book into the compass of a movie". This was achieved by "putting aside" all the components of the novel which did not directly relate to Harry and his journey.

The characters of Bill Weasley, Charlie Weasley, Ludo Bagman, Winky, Narcissa Malfoy, and Bertha Jorkins are all absent, as well as Dobby who was supposed to help Harry obtain Gillyweed for the second task.

_{I am unsure if Harry Potter: Behind the Magic and the behind the scenes extras from the Blu-ray are the same movie, it's difficult to find Behind the Magic online to check.}

In summation: Dobby and the rest of the characters were cut to focus more on Harry and his "thriller" journey in The Goblet of Fire.