Their new movie hinges on the fact that Butterfield's character Gardner was born on Mars during a Nasa mission and is struggling to adapt to life on Earth. "I'm a big science-fiction fan," says Butterfield, who previously worked in the genre on Ender's Game. "I think what's great about sci-fi is that everything that you see comes from technology that we already have, but just it's far more advanced. So they are almost giving glimpses into our future, whereas fantasy is totally, totally made up. This movie has a science-fiction backdrop, yet it deals with very small and natural feelings and emotions, which I think is pretty cool."

Britt and Asa in The Space Between UsBritt and Asa in The Space Between Us

Robertson liked that the film is about the distinctions between men and women (men are from Mars, indeed!), rather than the sci-fi angle. "Yes, it's about different beings," she says. "But it's like I'm sitting in therapy saying, 'He thinks this way and I just don't get it.' I've had a male therapist and a female therapist, and they give advice from two different perspectives. And it helps me! When speaking to a male about males you are like, 'Oh, that's how they think,' because you really don't understand that perspective."

For Butterfield, finding Gardner's physicality was key to bringing him to life in a way that was interesting and original. "There was the walk, the gravity," he says, "So I had to practice with weights around my ankle. I realised how difficult it was and would try to recreate that."

Meanwhile, Robertson's character Tulsa needed a steely edge. "I've been around people who have experienced what it's like to not have a real home or family," she says. "I think they carry this need to protect themselves emotionally, physically, whatever. That's what I tried to bring to Tulsa, this guarded chick who has had to provide for herself basically. But as you see her connection with Gardner grow, you see the layers unfold."

To get into the head of the characters she plays, Robertson finds inspiration in unexpected places. "I watched a really crazy episode of Dr. Phil today," she laughs. "It's always really fascinating stories. I think as an actor you're constantly observing human behaviour, and that show is just so good for the actor's mind, I guess. You wouldn't meet these people otherwise that are going through these crazy circumstances. So it's like news, but better!"

Watch the trailer for The Space Between Us: