Dan Stevens, who plays Haller, says the series steps deeply into the contemporary narrative of broken super-heroes, in which the traditionally heroic characters are played more as super-soldiers, dealing with their own version of post-war trauma.

"A lot superhero stories are about people coming to terms with their own makeup, the genetic or psychological, whatever," Stevens says. "With this one, it's just a more extreme version because his condition is such that he has been diagnosed as clinically insane, and for all intents and purposes he is."

Even when Haller is assured he's not insane, "he still doesn't just feel great straight away, and there's still a lot of quite strange things going on inside his head", Stevens says. "There are more issues that David has to deal with than just the one we really address in the first season, so the scope is vast."

In a genre where many of the iconic characters were conceived in the early 20th century and during the infancy of comic books, Legion is a relatively late addition to canon. He was created by writer Chris Claremont and artist Bill Sienkiewicz, and made his print debut in New Mutants #25, published in March 1985.

In the series, Stevens' co-star Rachel Keller was the first to sign on, in the role of Syd Barrett, a street-smart young woman whose power – if you could call it that – is that when her skin comes into contact with someone she and the person she touches trade minds. Though she becomes Haller's girlfriend, the pair can never touch.

"You're dealing with a group of people who work with such care and doing such quality work, and so, obviously, you're just on board and trusting to begin with," Keller says. "Then you look at the story and you look at the universe that we're going into, this beloved Marvel universe, and saying, well, why would we tell another story in here? What about this story is different and special?"

The show's creator Noah Hawley, whose most recent notable credit is the critically acclaimed US television version of Fargo, explained to Stevens and Keller he hoped the series could explore "the self work we all do, the self discovery, and how that creating relationships and falling in love," Keller says.

To create an on-screen chemistry, Stevens and Keller spent time together prior to filming. "Just to sort of break the back of this very strange relationship, really, and the sort of things that were required in establishing that relationship," he says. "There is no physical contact, so there has to be chemistry if you're not going to be kissing each other."

Stevens says the series explores the power of love to elevate you out of the particular set of circumstances you might find yourself in.

"Stuck in an institution and somebody like Syd walks into your midst, it's going to give you hope, and in David's case, it might be a false hope because it might be his brain playing tricks on him and she might not actually exist at all because he can't touch her," Stevens says.

"But he commits to this idea of loving her," he adds. "So we explore this sort of madness that all of us kind of experience, I think, when we fall in love, or if we've experienced anything like that, the transformative, transporting nature of love as a kind of madness. It certainly doesn't make their journey straightforward."

Production on the series, which was filmed in Vancouver, was unusually immersive, Keller says.

"You're filming in, like, a Stanley Kubrick mental hospital for a lot of the time, so there are moments when you're just walking through the hallways," she says. "There's the cinematographer, and there's the dresser, and you're like, am I in a movie of the making of Legion? Where am I?"

That sense of disconnection was amplified in part because many of the extras did not break character, Stevens adds.

"They're not necessarily featured that much, but if you watch closely, they're all in there and in amongst," he says. "And between takes people would sort of hold their reality. One lady had a delusion that she had a baby and she would just spend all day playing with this weird doll. There were definitely moments."

The series also sits neatly as a contemporary reflection of larger issues, Stevens says. "It's interesting time to look at a story where there is some kind of war taking place, there's some kind of conflict going on, and the idea of mutation and people that are different from ourselves," he says.

"There are two quite polar attitudes to those kind of people, [some] look at difference and embrace it and say, how can we use this for our betterment? The other attitude is to demonise it and run a mile and be very afraid of this difference. That, I think, underpins a lot of the X-Men chronicles and is a big part of Legion."

WHAT Legion

WHEN FX, Thursday, 7.30pm