First, some bad news: Even though Barty Junior is all over this film, we only get to see a little of the amazing David Tennant. Why? Because he's using Polyjuice Potion for basically the entire film to disguise himself as Mad-Eye Moody.
He's The Worst. Well, Almost The Worst.
Barty Junior is, to borrow a phrase, one bad hombre. He's a follower of Voldemort, and he's planted at Hogwarts to try to get to Harry—and get to him he does, after a long, convoluted series of events.
No one is really expecting Barty to be hanging around, since he was arrested and taken to Azkaban after Karkaroff exposed him as a Death Eater. However, it looks like he escaped…
The arrest was probably pretty embarrassing for his dad, who is a high up official in Ministry of Magic law enforcement. Guess this apple fell really far from the tree.
He's A Sadist
In a flashback late in the film, we learn that Barty Junior was responsible for torturing Neville Longbottom's parents. That intel definitely casts a new light on a scene that happens toward the very beginning, when "Moody" demonstrates the Cruciatus curse in front of Neville.
First off, it was bad enough that "Moody" was torturing a spider even though it was clearly bothering Neville (and everyone else). Hermione was totally right in calling the professor out on that from the get go.
When we learn the backstory on Barty and Neville's parents, though, that incident seems a thousand times worse. We realize that Barty (as Moody) was enjoying showing Neville the exactcurse with which he brutally tortured Neville's parents. That's just incredibly dark.
He Executes The Most Convoluted Evil Plan of All Time
Crouch Jr. (as Moody) starts putting his big plan into action early in the school year, and it's definitely an example of playing "the long game." He makes sure that Harry is chosen to compete in the Triwizard Tournament. Then, he feeds information to Harry to help him succeed at all three (quite dangerous) tasks.
The end goal? Ensuring that Harry is the one grabbing the Triwizard Cup at the end of the tourney. He wants Harry to get to the cup because it's actually a Portkey, and it takes our hero straight to Lord Voldemort.
(Now, you'd think that he could have just captured Harry and sent him straight to Voldemort at the beginning of the school year, but then there wouldn't be a whole movie to enjoy, would there?)
Harry escapes from that particular sitch, but not until the damage is done: Lord Voldemort is back in action, after stealing a bit of Harry's blood to help him get a body back.
He's Too Clever for His Own Good
Crouch Jr. has managed to maintain his cover up to that point, but when he gets Harry alone after the tournament, his excitement at hearing about Voldy's return betrays him:
MOODY/CROUCH: What was it like? What was he like?
MOODY/CROUCH: The Dark Lord. What was it like to stand in his presence?
HARRY: I don't know. It was like I'd fallen into one of my dreams . . . into one of my nightmares.
MOODY/CROUCH: Were there others? In the graveyard, were there others?
HARRY: Um…I don't think I said anything about a graveyard, professor.
Whoops. With his cover blown, Crouch decides to make the best of it and brag a little about his deviousness:
MOODY/CROUCH: Marvelous creatures, dragons, aren't they? Did you think that miserable oaf would have led you into the woods if I hadn't suggested it? Do you think Cedric Diggory would have told you to open the egg underwater if I hadn't told him first myself? Did you think Neville Longbottom, the witless wonder, could've provided you with Gillyweed if I hadn't given him the book that led him straight to it?
HARRY: It was you from the beginning. You put my name in the Goblet of Fire. You bewitched Krum, but…
MOODY/CROUCH: "But but..." You won because l made it so, Potter. You ended up in that graveyard tonight because it was meant to be so. And now the deed is done. The blood that runs through these veins runs within the Dark Lord. Imagine how he will reward me when he learns that I have once and for all silenced the great Harry Potter.
So, yeah, kind of like Voldemort in the graveyard, Barty Jr. gets a little distracted while patting himself on the back, and that gives the good guys enough time to intervene. Dumbledore and others bust in on Crouch just as he's about to kill Harry.
They take Crouch back to Azkaban…but, we'll say again, the damage has been done.
Crouch doesn't get a ton of air time in terms of the overall Harry Potter series, but man, for his role in getting Voldemort back to power, we'd say he made a pretty big impact on the overall story. For sure, his shenanigans are directly responsible for the shift in tone that the series takes moving into the next chapters.