On the set of Avengers: Age of Ultron , Chris Hemsworth and Chris Evans explain what’s next for their respective superheroes, and for the actors outside of the Marvel Studios universe. Part one of BuzzFeed News’ ongoing feature from the set of the film.

Marvel / Justine Zwiebel for BuzzFeed

LONDON — Thor and Captain America are ensnared in an epic battle on the set of Avengers: Age of Ultron. A legion of robot "Ultrons" — like the malevolent swarm from those haunting posters for the film, opening May 1, 2015 — are laying waste to a city within the (fictional) nation of Sokovia, under the control of the artificially intelligent titular villain. Civilians flee in terror, racing through streets littered with the husks of bombed-out cars and hunks of debris and shards of glass (or, rather, painted styrofoam and crumbly clear plastic) scattered along the ground. To evade the Ultrons' attack, Cap (Chris Evans) dives under an exploding pile of rubble, skidding along the ground on top of his trademark red, white, and blue shield. He then flips up onto his feet and kicks his shield into the air — just in time for Thor (Chris Hemsworth) to thwack it with his trademark hammer, slicing it through a pack of robots nearby like the most lethal and badass Frisbee ever. "Welcome to Sokovia," writer-director Joss Whedon, affecting a thick Eastern European accent, turns and says to BuzzFeed News during a solo visit to the set late in the film's production. "Miserable place. Everywhere robots." Except, of course, the robots aren't there. Nor is Cap's shield. Thor and Captain America have been battling the air. It's an overcast and weirdly cold July afternoon, on what had been a training facility for London’s Metropolitan Police force and is now a Sokovian city under siege. Every so often, a small pack of extras wearing gray reference suits and shiny silver robot heads will lumber through the scene so the visual effects team can know roughly where to add them in later. But when it comes time for our heroes to step in front of the camera, strike back, and (hopefully) save the day, all they can do is use their imagination and pummel an enemy that isn't there. Even Cap's shield is a CGI prop.

Jay Maidment / Marvel Chris Evans and Chris Hemsworth in Avengers: Age of Ultron

"The biggest thing is just not looking stupid," Hemsworth says later with a laugh, unwinding in his nearby trailer. "You've just got to remember there's another 30 different stages that are going to come into it before anyone will see that on screen." However, the Chrises — as they've often been called by those in the Marvel Studios Industrial Complex — do not appear all that concerned about seeming silly. In between takes, they joke around and giggle, lost in an inside joke neither can later recall (or, perhaps, prefer not to share on the record). In fact, if they weren't respectively dressed in the gleaming armor of an Asgardian warrior and the complex uniform of an American super-soldier, you might be forgiven for mistaking them as old friends goofing around in their backyard playing superhero, instead of two of the stars of the sequel to the third highest-grossing movie of all time. "We're buddies," Evans says later with a shrug in his own trailer. "We've been on a little bit of a ride doing these movies. You make really good friendships. It sounds cliché to say: 'We all get along so well!' We really get along phenomenally well. It's like summer camp." If the set of Avengers: Age of Ultron is like a summer camp, then that would make Evans, 33, and Hemsworth, 31, something like veteran camp counselors. This is the fourth Marvel Studios movie for both actors; other than 2012's The Avengers, each have headlined two of their own Marvel Studios films, to increasing box office returns. They have in turn made dramatically different choices about how they've built their careers outside of Marvel's familiar campground. And to stretch this metaphor even further, neither actor can stay at camp forever.

Marvel / Adam B. Vary for BuzzFeed

Earlier in the day, the newest additions to the Avengers cast — Aaron Taylor-Johnson (as the super-fast Pietro Maximoff, aka Quicksilver) and Elizabeth Olsen (as the immensely powerful Wanda Maximoff, aka Scarlet Witch) — approach their action scenes with the intense, eager concentration of rookie recruits. When Evans first rehearses his ground-skidding stunt, by contrast, he casually moves through each beat at half speed, and when he's finished, he flashes a wide grin, and jokingly announces to no one in particular, "I'll be in my trailer!" "The cast — they were very trusting the first time around," says Whedon. "But this time, [there's an] ease and rapport, and just the fun that we have. Everybody's got franchises and babies. Everyone's very comfortable with themselves."

Marvel Chris Evans in 2011's Captain America: The First Avenger

It's the kind of comfort that can only come with years of experience of what it's like embodying a Marvel superhero. "In the beginning, you're just so grateful," says Evans. "You're very timid. You're lucky you got invited to the barbecue, [and] you just don't want to get in the way. Now, you feel a little more at home, and it feels a little more like a collaboration that you contribute to. If anything's changed, it's just my feeling of ‘welcomeness.’ Is that a word, welcomeness? We're going to make it a word, fuck it." That familiarity, however, can cut both ways — there are only so many places one can take a character who is essentially an immortal god. "You know the rhythm of it," says Hemsworth. "What becomes a challenge is trying to not repeat the same thing all the time. So you've got to work that bit harder to see what else you can do with the character." For Thor in Age of Ultron, "it was just lightening up a bit," Hemsworth continues. "It gave us room to kind of make him a little more grounded and human and have him in some civilian clothes and mixing it up at a party. I want to do those scenes more. It's what I loved about the first [Thor film]. There was an innocence and naïveté to him, which as he matured into the king of the second [Thor film] — or the rightful king — we sort of lost a little bit of that. That was that story." The biggest difference for Thor in Age of Ultron is the absence for the first time of his scheming brother Loki (Tom Hiddleston), a character so outrageously popular that he has in many ways overshadowed Hemsworth's work as Thor. Loki was last seen at the end of 2013's Thor: The Dark World sitting on the throne of Asgard, masquerading as King Odin (Anthony Hopkins), and that is where he will remain for the time being, as Age of Ultron focuses the action on Earth.

Marvel Tom Hiddleston and Chris Hemsworth in 2013's Thor: The Dark World

"I mean, [I] loved working with Tom," says Hemsworth, "and that relationship is so much fun. But that also kept us in a bit of a box, because, you know, three films on one relationship, how much further can you go? What else can you do different? It was nice to have a whole different sort of motivation." So what is that motivation this time out? Hemsworth won't say outright, of course, but he does offer a few tantalizing details. The film's central plot tracks the growing power of Ultron (voiced and performed via motion capture by The Blacklist's James Spader), a sentient robot created by Tony Stark (Robert Downey Jr.) with the intent of giving the Avengers a breather from constantly fighting the forces of evil. But if Loki's machinations brought the Avengers together in the first film, Ultron's global ambitions — he's not a big fan of humanity — would appear to split the squad apart. "Whereas the first [Avengers] was [about] the journey of the team, I think, this time, you have six or seven things going on, which I was really impressed by," says Hemsworth. "I like where Thor has been taken in this. He sort of sees a whole other side to what's going on in the conflict, and another potential threat. And he kind of segues a bit and has his own little sort of journey." That fractured storytelling portends a momentous shake-up in what we've come to know as the Marvel Cinematic Universe, a sense that’s made all the more palpable in the trailer for the film. "The Avengers films, ideally, in the grand plan are always big, giant linchpins," says producer and Marvel Studios chief Kevin Feige on the Age of Ultron set. "It's like as it was in publishing, when each of the characters would go on their own adventures and then occasionally team up for a big, 12-issue mega-event. Then they would go back into their own comics, and be changed from whatever that event was. I envision the same thing occurring after this movie, because the [Avengers] roster is altered by the finale of this film."

Marvel / Adam B. Vary for BuzzFeed

Any overhaul in the makeup of the organizing force behind the MCU could weigh especially heavily on Evans' Captain America, who in his most recent solo film, last spring's Captain America: The Winter Solider, was instrumental in bringing down S.H.I.E.L.D., the organization that had unified the Avengers in the first place. "He's still looking for a home, probably a metaphorical home," says Evans. "He's always felt comfortable as a soldier. And he likes structure. He works well taking orders. But when that dynamic turned on him, he's now left to depend upon his team, the Avengers. There really is no one above them telling them what to do. They're kind of having to operate independently. So there's a lot of leaning on one another, but there really isn't a kind of clear chain of command. And I think Cap looks for that. I think he's looking to understand where he belongs, not just as a soldier, as Captain America, but as Steve Rogers, as a person."

Zade Rosenthal / Marvel Chris Evans in 2014's Captain America: The Winter Soldier

Of course, Steve Rogers was injected with a super serum that turned his body into a perfect physical specimen, and, for Evans, maintaining that physique on set has not gotten any easier. "I found out on the first movie, typically a lot of the stunt stuff is towards the end of the film," says Evans. "And that's when you're in the suit. And when you're in the suit every day for a month, you just drip sweat, and you shed weight. So you try to get as big as you can in the beginning, and hopefully they've tailored the schedule so the stuff where you're walking around in a T-shirt is early. You know, we're in the last three weeks right now. I feel like I've lost like nine pounds in the past couple weeks. By the time you finish a [Marvel] movie, you're kind of back to your starting weight." For his part, Hemsworth has faced a slightly different issue with transforming his body into that of a superhero. "It's not so much the staying in shape," he says, chuckling. "Once you get to the shape, maintaining it — it's work, but it's less work than trying to get it off to do another role, or to put it back on when you start from scratch. That's been tough." Hemsworth cites the film In the Heart of the Sea, a harrowing nautical adventure he shot between Thor 2 and Avengers 2 in which he played a sailor stranded in the open ocean for several months. "Getting down to the weight I did for [that movie] was really exhausting," he says. "It was a good six, eight months of being on some sort of restricted form of diet, and some high cardio sort of workout. And then the moment we wrapped that, literally the day of, was, OK, eat more and go the opposite direction, because Avengers is three months away. So you feel like an athlete more than an actor." He laughs. (Hemsworth laughs a lot.) "It's been insane over the last few years. But it's great. I love it. I like just staying active anyway, and find this sort of extreme form of it fun. It's a challenge."

Marvel Chris Hemsworth in 2015's In the Heart of the Sea

It's a challenge Hemsworth will almost certainly face again, since he is clear that his next major film "won't be Thor 3 — that will be the following year." (On Oct. 28, Feige did indeed announce that the third Thor film, Thor: Ragnarok, would open on July 28, 2017, just over a year after Captain America: Civil War opens in May 2016.) Hemsworth says he hopes to get an adaptation of Steve Earle's novel I'll Never Get Out of This World Alive that he’s also producing off the ground, and he's also committed to shooting his first outright comedy, a reboot of sorts of the National Lampoon's Vacation series, followed by a sequel to the 2012 Kristen Stewart vehicle Snow White and the Huntsman that will reportedly focus almost entirely on Hemsworth's Huntsman character. Keeping busy as a leading man has indeed been a career priority for Hemsworth since he was plucked from relative obscurity to play Thor. He also starred as rakish race car driver James Hunt in Ron Howard's well-received 2013 drama Rush, and he's headlining Michael Mann's cyber-thriller Blackhat, due to open Jan. 16, 2015.

Marvel / Adam B. Vary for BuzzFeed

By comparison, Chris Evans has maintained a decidedly lower profile after landing his Captain America gig. His only leading role outside Marvel Studios has been in the rapturously reviewed dystopian sci-fi thriller Snowpiercer, a film that was something of a sensation overseas but received a limited, specialty release in the U.S. Unlike Hemsworth, however, Evans had been working as a leading man throughout the 2000s — he even played another Marvel superhero, the Human Torch, in two Fantastic Four movies for 20th Century Fox (which the studio, which owns the movie rights to the Fantastic Four characters, is currently rebooting with a brand-new cast). Still, when Evans gave interviews last spring in which he said, "When I'm done with this Marvel contract, I'll take a little break from acting" and "I can't see myself pursuing acting strictly outside of what I'm contractually obligated to do," the immediate headlines were that Evans was going to outright retire from acting once he was done playing Captain America.

RADIUS-TWC Chris Evans in Snowpiercer