Michael Fassbender: At some point everybody—well, not everybody, but a vast majority—has felt displaced for one reason or another. Going to high school, some people have glasses, other people have certain religious beliefs or sexual orientation. For whatever reason, they feel ostracized. I think the great thing about the X-Men series is that people everywhere can relate to it.

Fassbender sips from his pint of Sussex Bitter Ale as his close friend and costar James McAvoy takes over the job of explaining the appeal of their new $200 million–plus film, X-Men: Days of Future Past—both for audiences and for these two accomplished actors. It's early still at the Lord Stanley in North London, but in true pub-sermonizing fashion, McAvoy becomes instantly animated.

James McAvoy: People have been asking me a lot, "Why have superhero movies become so popular recently?" And they go, "Is it because they've become really serious and blah, blah?" I'm like, "No, it's not." For thousands and thousands and thousands of years, we've been telling stories about superheroes. Norse gods, mythical fuckin' Greek gods, Roman gods—Hercules? It sounds like fuckin' Wolverine, you know what I mean? We have been telling stories about superheroes and super-villains forever. This isn't some new thing that we now do and sell out for.

MF: That was good. [Claps] I'm in admiration.
JM: I just spent yesterday junketing.
MF: I missed out.
JM: By the end of junketing, you've got some really good bullet points.
MF: It sharpens the spear.
JM: It really does.
DETAILS: Mind giving us another bullet point?
JM: I've been watching X-Men cartoons since I was 14. I'm a huge fan. And that is true, but it's also something I said about 4 million fucking times yesterday.
MF: I get pissed off at myself because you're like, "God, I've said that so many times now," and you start to feel like you're insincere. But actually, you're just looking for a truthful answer. There's nothing devious or preconceived—for me, anyway.
JM: The only time that I am dishonest . . . when I make shit up or when I deflect . . . is when I'm being asked something that is—
MF: About somebody you don't like?
JM: Totally. Then I'm a lying bastard. I'm like, "Michael, he's one of the finest."
MF: He's like, "He's such a good guy."

• • •

first-class friends

If life were as poetic as they would like it to be, McAvoy and Fassbender would have bonded while making Band of Brothers back in 2000, playing fresh-faced G.I.'s in the Tom Hanks–Steven Spielberg HBO series, but they didn't shoot any scenes together. Instead, their bromance blossomed a decade later, in the walk-up to 2011's X-Men: First Class, by which time they had developed lengthy résumés and mutual respect. "Before Michael and I even met, I was already willing to go with him and be open to him, because I was like, 'This guy's fuckin' brilliant,'" McAvoy says. "Not to be too fuckin' up your ass or anything like that, but the thing that elevated First Class for me was working with you."

"I had had admiration from a distance," Fassbender says, adding that the connection crystallized during the audition process. McAvoy, who was director Matthew Vaughn's first choice to play Charles Xavier, tested with all the actors up for the part of Erik Lehnsherr, a.k.a. Magneto, in an effort to find the perfect chemistry. "James came in to do the screen test with me," Fassbender says, "and from there, there was a respect and a friendliness between us. But then as it developed, there was more trust, and you realize that the other person's got your back. Then the trust becomes deeper and it goes somewhere else, for sure."

It progressed from giving each other notes to writing and rewriting lines for each other to what McAvoy calls "a relationship that was comfortable, honest, and allowed us to be able to be vulnerable in front of each other as friends and ask, 'What the fuck do you think about this?'" Not surprisingly, it's the fraternal relationship between McAvoy and Fassbender—Charles and Erik—that helped make the film a sleeper blockbuster, taking in more than $350 million worldwide.

As deep and abiding as it is, the friendship is defined by the differences in their lives in a way familiar to many men in their thirties. One is settled down, with a wife and a child, ensconced in a quiet neighborhood (sleepy Crouch End, where McAvoy lives with his wife of eight years, Anne-Marie Duff, and their son, who turns 4 in June). The other is still in the messy bachelor pad he's had for years (Fassbender's been meaning to spruce up his apartment in an edgy corner of Hackney ever since he moved there in his twenties). Both have demanding jobs. So they text, send each other videos, and are thick as thieves when working, as they were while filming X-Men: Days of Future Past in Montreal. And off set—let's just say that their outings still feel like something of an occasion.

Hence the pints, hence the pub—the Lord Stanley. Tucked away in the boho borough of Camden, the airy gastropub splits the distance, geographically and otherwise, between the two friends.

There's another thing that makes this afternoon drink-up special: It's Fassbender's birthday, his 37th. While he is going out to dinner later this evening, he plans to take it easy. He doesn't always, as suggested by the tabloid reports of him carousing with a supermodel at a club the night before. He alternates sips of Vita Coco with his first round, which comes courtesy of his good mate.

"It's not a contrivance at all. I love the guy," McAvoy says, turning to Fassbender. "I do mourn your absence sometimes when I'm working with lesser dudes."

• • •