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Captain America sets the pace. He starts with a slow walk, eyes fixed forward, before breaking into a light jog. The others follow suit, from reliable Cap fixtures like Bucky and Falcon to full-fledged Avengers like Scarlet Witch and Hawkeye. Even Ant-Man joins the pack, fresh off his stint breaking into high-tech facilities and traveling through the Quantum Realm in his first solo movie.

Soon, Hawkeye grips his bow with battle-ready purpose, signaling an increase in the intensity and speed of the action at hand. With Cap leading the way, these titanic heroes of the Marvel Cinematic Universe run forward, chasing a black SUV equipped with a camera, filming what directors Joe and Anthony Russo refer to as the "splash page" shot of Captain America: Civil War.

"As a kid I would just go through [comics] and look at who was fighting who," says Joe, speaking on the blistering hot Atlanta set of the new Marvel movie. "I'd stand there in the store for fifteen minutes until the guy told me to buy the book or get out. You'd just study it, and so this sequence [we're shooting now] is our live-action splash panel or double panel -- it's a big, epic sequence."

The word "epic" certainly captures the feeling as Cap and his costumed allies run forward on what will ultimately become a massive airport runway when the final movie comes together. But the enemies they're rushing to battle aren't your traditional Marvel bad guys. In fact, they're Marvel heroes -- and even though many of the characters aren't on set at the moment, fans already know their names: Iron Man, Black Widow, Vision, War Machine, and newcomer Black Panther.

Forget Loki and Ultron. In Civil War, the Avengers face their mightiest foe yet: Each other.

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The thirteenth film in the Marvel Cinematic Universe, the third film in the Captain America series, and the first missile launched in Marvel's Phase Three, Civil War breaks the MCU in half, with star-spangled super-soldier Steve Rogers and genius-billionaire-playboy-philanthropist Tony Stark right at the core of the conflict.

"This isn't a character struggle," says Chris Evans, now logging in his sixth turn as Cap, including his cameo in Thor: The Dark World. "This is kind of an execution struggle — how things should be done. [Cap] respects Tony as a man, and I'm sure he respects me as well. We just each have different emotional concerns."

That's a very diplomatic way of putting it, given the impending battle at the heart of the aforementioned splash page sequence, not to mention the numerous beat-downs between Steve and Stark in previous Civil War trailers. The reason behind the split between the two heroes remains somewhat enigmatic, even now. No, there's no Superhero Registration Act, unlike the Civil War comic books the movie takes its name and several cues from. But the world nonetheless expects some regulation when it comes to its superheroes.

"The Avengers have been operating independent of any government restriction, so I think that makes plenty of people nervous," says Evans. "I don't think I'm giving anything away by saying what happens is certain governments expect a little bit of a change."

In the comics, Cap and Iron Man's feud stems from an explosive incident in Stamford, Connecticut, one that claims the lives of countless innocent bystanders, including children. The film will not replicate that exact sequence, but co-writer Stephen McFeely confirms that a similar kind of incident will launch the conflict at the heart of the film.

"It's not Stamford," he says, "but we'll have an incident that will force the governments of the world to go, 'Wait a second. Let's talk about the laundry list of things that we're not happy about. Let's finally do something about that. We think you guys need some oversight.'"

"It plays in an interesting way into people's reactions to these movies," adds Christopher Markus, who has written all three Captain America movies alongside McFeely. "We dropped Helicarriers on Washington, D.C. You do this, and it's great, but eventually you have to go — if this is a realistic world, somebody's going to go, 'Stop dropping Helicarriers on my fucking head!'"

The question about superhero regulation has been on the Marvel Universe's mind in both Avengers movies and was at its most heightened state in Captain America: The Winter Soldier, the film that saw the dissolution of the corrupted S.H.I.E.L.D. agency, thanks to the efforts of Cap and his allies. Civil War marks a natural extension for Rogers, then, as he continues to challenge the modern world's leadership and authority -- except this time, even Cap doesn't know if he's on the right side of the issue.

"I think this is the one time there's a conflict where his compass doesn't know where to point," says Evans. "I think he handles conflicts well because he knows what's right and he knows the right thing to do, but sometimes that's hard, because it may effect certain people and it may butt with what other people believe, but at least he knows his own mind. I think this [movie] is one of the first times he doesn't know -- and I think when you're kind of aimless, I think that's just terrifying."

Luckily, Cap isn't alone. Some of his most trusted companions are on the other side of the battle lines, but not all of them. Take Sam Wilson, played by Anthony Mackie, for instance. He remains one of Cap's closest fly-or-die allies, even in the thick of an impossible conflict.

"I think their relationship carries over from when I was introduced in [Winter Soldier], and you saw a little bit of it in Avengers," Mackie says of Steve and Sam's dynamic. "There's definitely a confidence and respect with the two of them, and you get to see more of that."

But it's Steve's oldest friend that provides some of the film's most intense drama: Bucky Barnes, played by Sebastian Stan, who seemingly broke free from his brainwashed ways at the end of Winter Soldier. That's where Bucky's story in Civil War begins, according to Stan: "It's very much a big struggle, figuring out what his life has been about and what he's really been up to." It's an equally big struggle for Rogers, who feels an intense sense of loyalty to his old friend, even if some of his colleagues -- like, say, Tony Stark -- view Bucky differently.

"[Bucky] is a huge piece of his history and a huge piece of his struggle," says Evans, "not just to have someone that he can connect to on a friendship level, but just the guilt he must have. 'I let you go. I'm sorry.' Just the survivor's guilt element, there's plenty to play with."

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Indeed, "plenty to play with" is the name of the game in Civil War, a film loaded with so many characters that the two Avengers movies look like indies in comparison.

"It's a sprawling film, no question," says Joe Russo. "This is much, much bigger than Winter Soldier. In fact, it's probably bigger than anything [Marvel] have done to date — just in scale of character, without question."

"It's also very complex on that level," adds Anthony, "because while these characters have had conflicts before, this movie takes it to an entirely different level. So you're taking your protagonists and you're turning them into antagonists for one another, and that's a very complicated process for these beloved characters."

Even with all the characters in play, there's only one specific hero with his name in the title: Captain America. The Russos, McFeely, and Markus all testify to their desire to create fully realized arcs for the individual characters in the movie, but at the end of the day, this is Cap's show, brought to life by the same people who turned Winter Soldier into one of the best reviewed superhero films of all time.

"We're taking Cap to a place in this movie that he's never gone before," says Anthony. "How do you take this guy who began where he began, and goes that great arc that he's had, and yet still take him to a place he's never gone before?... We found a way to really get at the heart of who Cap is and shake his foundation, and push him somewhere that's going to surprise a lot of people."

"Balance is absolutely the challenge," says McFeely. "We don't fool ourselves into thinking everybody is going to have... we're not going to split all the roles and lines and screen time into fifteen parts. It is a Captain America movie, and lined up on the opposite side of him, first and foremost, is Tony Stark."

Indeed, if Steve Rogers is number one on the call sheet, there's an Iron Man waiting for him on the opposite side of the poster -- or the splash page, as it were.

TOMORROW: Inside Iron Man's Side of the Civil War.

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Captain America: Civil War hits theaters on May 6.