The two actors are longtime friends off screen and now play even longer-time lovers in Marvel’s latest tentpole.

EW: If you’re going to play immortal partners with an on-again, off-again relationship of 7,000 years, it helps if you have some history.

Real-life friends Gemma Chan and Richard Madden star as Sersi and Ikaris in Marvel’s Eternals, an ambitious epic from Oscar-winning Nomadland director Chloé Zhao. The film (out Nov. 5) follows a group of ageless superhumans as they protect mankind through the centuries, with a starry cast that includes Kumail Nanjiani, Lia McHugh, Brian Tyree Henry, Lauren Ridloff, Barry Keoghan, Don Lee, Salma Hayek, and Angelina Jolie. But it’s Sersi and Ikaris’ tender romance that anchors the film, stretching from modern-day London all the way back to their first arrival on the planet in about 5,000 B.C.

For EW’s Fall Movie Preview, we sat down with Madden, 35, and Chan, 38, for insight into how they brought their undying love story to life.

ENTERTAINMENT WEEKLY: How did you two first become friends?

GEMMA CHAN: I think just socially.

RICHARD MADDEN: Because we have so many friends in common.

CHAN: It’s a small world in London.

MADDEN: Somehow we ended up just spending lots and lots of time together. I think we ended up not having jobs at the same time, and that helped, like, “Oh, great, you’re also not working.” [Laughs] And we never ever talked about work!

CHAN: No!

MADDEN: So it’s quite funny that we ended up then working together, when we’ve never really acknowledged each other in the industry or talked to each other about work.

How did you react when you both were cast in Eternals?

MADDEN: Over the moon. I mean, we’ve known each other for over 10 years now, so if you’re going to go to work and get up and see someone at 4 a.m. every day, it makes it a lot easier if you like them. Because we’ve known each other so long, we already have this trust and rapport. We know how to push each other’s buttons. We know how to make each other laugh. We wanted to bring out the best in each other, so it makes it just a nice, organic process — rather than meeting someone for the first time and being like, “Hey, you’ve got to make out now.”

Gemma, you’ve joined the MCU before, as Minn-Erva in 2019’s Captain Marvel. What was your reaction when they asked you back for Eternals?

CHAN: I was not expecting to be asked back so soon. When I spoke to [Marvel Studios president] Kevin Feige and he said, “We would love to have you back,” I thought he was just being nice. [Laughs] Like, maybe at some point in the distant future, if at all. But it was a nice surprise. And to be working with this one was a treat. Obviously I wasn’t going to turn that down.

What was it about this millennia-spanning epic that first hooked you?

MADDEN: For me, it was the fascination of when you’ve got a character that’s thousands and thousands of years old, what is it about them that makes them want to keep living? What is it about the world around them that they find joy in?

CHAN: And I was excited about playing this character where I haven’t really seen a superhero like her before. She’s a free spirit. She’s not your typical warrior-fighter kind of superhero. And I was interested in this mix of characters as well, them coming together and [figuring out] what they’re like as a slightly dysfunctional family.

Richard, your Game of Thrones brother Kit Harington also stars in Eternals. How was that reunion?

MADDEN: Just great. I mean, I’ve been friends with Kit for many, many years, and sadly we don’t get to do a lot in the film together. But I get to have one moment with him where we say hello, and it’s nice to be on set with someone you know and trust and have known for a long time. On set, it was kind of like hanging out with your friends, and then it’s like, “Oh, right, we’ve got to do acting now? Okay.”

For each of you, what was the biggest challenge of playing these immortal, seemingly untouchable heroes?

CHAN: For me, it was finding moments to bring truth amongst the craziness of a production of this size, with everything that’s going on and the challenges of shooting on location. Finding moments where you could be still, or just have a really intimate, small moment—that kind of grounds you through it.

MADDEN: I’d say the same. It’s finding these moments of reality amongst laser eyes and fighting monsters and being thrown into rocks and upside down. These characters are superheroes, but they’re also souls, and they have really complex feelings and relationships that have spanned a long time. We tried to find the truth and honesty in those moments, while wearing superhero costumes and diving off cliffs and stuff.

What was the superhero training process like?

CHAN: [To Madden] Oh my gosh, you have to do so much.

MADDEN: Because I’m flying all the time in the movie, I’m on these wires in the studio and in different locations. It’s just very hard because you have to pretend like it’s effortless, when actually you’re in this kind of corset harness, and your legs are up here, and your blood’s all rushing to your head, and you have to swan in as if you’re really cool — and actually you’re screaming on the inside. [It helps] having a friend to say, “Hey, does this look dumb?” And they’ll say, “Yeah, you look like Peter Pan.”

That helps, having someone to tell you where you fall on the scale of dumb to cool.

MADDEN: Gemma is great at telling you if you look dumb.

CHAN: [Laughs] Hey, you do the same for me!

MADDEN: But she always looks cool all the time, so that’s easy. [Laughs]