How does "Mad-Eye Barty" compare to Mad-Eye Moody?

Question

At the end of Harry Potter and the Goblet of Fire, we learn that who we thought was Alastor "Mad-Eye" Moody, was actually Bartemius "Barty" Crouch Jr, who impersonated him through the use of Polyjuice Potion, for the entire book.

We've never met Mad-Eye Moody at that point.

He fooled us all — the readers — just like he fooled Dumbledore, an old friend of Moody's, so he must have imitated the real Mad-Eye quite well.
He may have been acting out of character when he punished Draco by turning him into The Amazing Bouncing Ferret (although that's not certain); it probably wasn't out of character for him to teach the Unforgivable Curses to fourth year students.
What was completely out of character, and what tipped-off Dumbledore in the end, was how he separated Harry from the rest just after having returned from the graveyard, with the dead body of Cedric Diggory in tow.

From what I remember, in the subsequent books, the real Mad-Eye Moody doesn't seem much different from the fake one. Even though we've just met him, it feels as if we've known him longer, but that wasn't Moody, that was Barty. Harry's (and our) history with him doesn't go back to the Triwizard Tournament — that was Barty.

So how does "Mad-Eye Barty" compare to Mad-Eye Moody? What mistakes (if any) did he make?

Answer 1

It seems Barty Crouch Jr compared quite favorably to Mad-Eye Moody, disclosing very few slip ups:

‘It hasn’t been easy, Harry, guiding you through these tasks without arousing suspicion. I have had to use every ounce of cunning I possess, so that my hand would not be detectable in your success. Dumbledore would have been very suspicious if you had managed everything too easily.'

_{Goblet of Fire - page 587 - Bloomsbury - chapter thirty-five, Veritaserum - Barty Crouch Jr as Mad-Eye Moody}

Goblet of Fire reflects that Voldemort's plan to use Barty Crouch Jr to impersonate Mad-Eye Moody was executed very, very well. I noted Barty made very few canon-based slip ups while impersonating Moody. The ones that come to mind are:

• Being caught on the Marauders Map as "Bartemius Crouch" by Harry when Barty Jr was stealing potions ingredients from Snape's stores; I wouldn't be surprised if Moody would have found a way to avoid this, even not knowing of the existence of the map and maybe even if it involved using the Imperius Curse to get someone else to procure the potions ingredients he wanted. Barty Jr's failure to plan for this contingency definitely breaches Mad-Eye Moody's mantra of "CONSTANT VIGILANCE!" Crouch Jr should have always maintained the possibility that there was some kind of magic within Hogwarts that, under the right circumstances, could identify who he really was (that's what Moody would have done).
• Barty Jr was not prepared at all times with, say, the antidote to Veritaserum¹ to take before Dumbledore arrived from the maze and stunned him, and then ordered Snape to give Barty Jr the truth-telling potion.
• Barty Jr did not have an Invisibility Cloak on him at all times, as far as we know; Harry and Snape, at the very least, would have recognized an Invisibility Cloak by sight, which may have aroused suspicion. If I recall correctly, Mad-Eye Moody actually had two Invisibility Cloaks, but lost one to Sturgis Podmore when Sturgis was put under the Imperius curse by Death Eaters (please correct me if I'm wrong), so it goes to show that even Moody can make mistakes. Anyhow, Barty Crouch Jr also had an Invisibility Cloak, and I don't recall that it was mentioned as having been destroyed or damaged to the point of being unusable.
• As you yourself mention in your question, Barty Jr did not obey Dumbledore's order to keep Harry within Dumbledore's sight at the end of the maze portion of the Triwizard Tournament. Mad-Eye Moody would not have taken Harry away from Dumbledore's protection; Barty Crouch Jr did, piquing Dumbledore's suspicion of him.

¹_{And before everyone gets all up in arms about the impossibility of blocking the effects of Veritaserum:}

_{"Veritaserum works best upon the unsuspecting, the vulnerable and those insufficiently skilled (in one way or another) to protect themselves against it. Barty Crouch had been attacked before the potion was given to him and was still very groggy, otherwise he could have employed a range of measures against the Potion - he might have sealed his own throat and faked a declaration of innocence, transformed the Potion into something else before it touched his lips, or employed Occlumency against its effects. In other words, just like every other kind of magic within the books, Veritaserum is not infallible. As some wizards can prevent themselves being affected, and others cannot, it is an unfair and unreliable tool to use at a trial."}

Wayback Machine - JKRowling.com (Original J.K. Rowling site) Note: This site can frequently be difficult to access; it's kind of hit or miss whether it'll pull up.

Answer 2

Barty Crouch Jr. acted very much like the real Mad-Eye Moody.

J.K. Rowling addresses this directly in a question about whether Mad-Eye Moody is actually the real one and whether up to something, since he’s still acting like Crouch-as-Moody had been. She said that Crouch had indeed acted just like the real Moody.

SnapesForte: Is Mad Eye Moody the real Moody this time? And if he is, is he up to something fishy? Because he's acting too muhc like Crouch jr - sniffing food etc

JK Rowling replies -> It's the other way around - Crouch Jnr. acted just like the real Moody.
_{^{- JK Rowling's World Book Day Chat (March 4, 2004)}}

As far as if Barty Crouch Jr. made any mistakes in his portrayal of Moody other than the one mentioned in the question about him separating Harry from everyone after he returned from the graveyard (not including the ways he’d have to diverge from the real Moody due to his real plan) that’s less clear - but there would be very few of them, since he had even Dumbledore fooled.

It would also be illogical to classify any mistakes in defending himself as unlike the real Moody, since even Moody wasn’t always prepared for every possible situation - he had been successfully captured by Barty Crouch Jr. and Wormtail, then kept in a trunk for months without him being able to escape.

Answer 3

Barty was more cruel than Moody (at least in the movie at some points) and also wasn't nearly as vigilant (like when he didn't notice the mirror was warning him Dumbledore and Snape were approaching). He also didn't seem as paranoid as Moody was (Moody weaponized garbage cans outside his house).

One last thing, in the movie Crouch accidentally mentioned that Harry was sent to a graveyard before Harry said that, which gave him away to Harry (something the real Moody would never do).