When Ben Foster admits to using performance-enhancing drugs in preparation for his latest film, he says it so casually that it takes a moment to register the confession. “I don’t want to talk about the names of the drugs I took,” he explains, sitting across from me in blue jeans and white shirt in a Paris hotel suite. “Even discussing it feels tricky because it isn’t something I’d recommend to fellow actors. These are very serious chemicals and they affect your body in real ways. For my own investigation it was important for me privately to understand it. And they work.”

Foster – who is 34, with short, blond hair that stands upright, an incipient beard and moustache, and the belligerent handsomeness of Kirk Douglas – has long been renowned as an actor for whom merely going the extra mile would be just plain lazy. During the filming of Lone Survivor, he ate handfuls of dirt to get an idea of the hardship endured by Navy Seals under fire from the Taliban. He used glaucoma eyedrops to dilate his pupils when playing a crystal meth addict in Alpha Dog. His role as a homeless, disabled veteran in Rampart was scarcely more than a cameo, yet he still spent time beforehand living rough on the streets of Los Angeles. (“I was pissing my pants like everyone else there,” he recalls.)

So it makes sense that, having been cast as American cyclist Lance Armstrong in Stephen Frears’s new film The Program, he might choose to dabble in the performance-enhancing drugs that the athlete used to propel himself to seven consecutive Tour de France titles. But if it’s acceptable to take drugs for The Program, I ask, why not use crystal meth for Alpha Dog? “It’s a good question,” he replies. “In all transparency, I rolled with those guys on Alpha Dog and I bought their drugs but you have to ask yourself how far you can go and still come back. I had been losing friends to crystal meth. The proximity to it was enough that I didn’t need to take that door.”

Facebook Twitter Pinterest Ben Foster in the trailer for The Program

Foster remarked last year that he “lost his fucking marbles” while making The Program. “There’s a fallout,” he tells me. “Doping affects your mind. It doesn’t make you feel high. There are behaviours when you’ve got those chemicals running through your body that serve you on the bike but which, when you’re not …” He shakes his head. “I’ve only just recovered physically. I’m only now getting my levels back.” I ask how the “fallout” manifested. “I don’t know how to separate the chemical influence from the psychological attachment I had to the character. If it’s working, it keeps you up at night. This is losing your marbles, right? They’re definitely rolling around. They’re under the couch but they’re retrievable.” Have there been times on other movies when he feared they might not be? “Oh, I don’t wanna talk about those,” he says, looking away bashfully.

The actor knew very little about Armstrong when Frears first approached him to play the part. “I knew he started Livestrong,” he says, referring to the charity that Armstrong established in 1997 in the wake of his own recovery from testicular cancer. “I knew he was the greatest at one point. I knew he was considered a liar.” Prior to immersing himself in a punishing training regime and spending time on the USA Pro Challenge tour in Colorado, he had no preconceptions. “I was a really clean knife.”

Foster’s take on Armstrong, who declined his request for a meeting, is complicated. “On one hand, he’s a lying doper who tricked the world. On the other, he’s a young man who faced cancer. It changes you. And when you go to war it changes you. That’s what Lance did – he went to war with his body. That shifts your consciousness.

“He started training within a culture that was doping: you’d have to go down 18 riders to find a clean one. He survives death, the story catches fire and he recognises that.

“He’s a smart man. He says, ‘I can do some good with this.’ He raised half a billion for cancer research. We just don’t like him because he was Jesus Christ on a bicycle. We’re mad he came back from the dead, saved the sick and then turned out to be full of shit. And we’re punishing him because he didn’t apologise in the way we’d like. Americans love a good apology. He wouldn’t do that.”

Mimicry wasn’t on the cards for Foster. “The word I’m leaning toward is ‘infected’. I tried to infect myself with him. Get him into my system.” That’s apt given the disease imagery in the movie: at one point, Armstrong describes the accusations about his doping as “malignant cells” trying to colonise him. “It was important to get his gait, his riding style. I talked to people who had the aerodynamics of his body on a computer system so I could get the hump in the back, the heels slightly out. It’s almost a duck pedal. It’s not a delicate ride. It’s violent, which is also why he’s such an exciting rider. It’s like he wants to break the bike.”

Facebook Twitter Pinterest Foster as Armstrong in The Program. Photograph: Allstar

A cautious admiration emerges when Foster discusses Armstrong. “Belief and will got him through, not dope,” he insists. “Dope certainly helped. Had he not been doping he wouldn’t have won. But his greatest attribute is his ability to believe he’s a winner.”

He makes the connection to his own profession before I can. “That righteousness, that self-belief, could be considered akin to acting. The best acting. It’s not lying, it’s belief.”

Foster would know. No one does authenticity quite like him. Plucked out of his Iowa upbringing at the age of 14, his first regular acting job was on the Disney series Flash Forward. There was a muscularity and thoroughness to the television performances that followed—as Eli, an exuberant high-school kid with learning difficulties in Judd Apatow’s Freaks and Geeks, or Russell, the bisexual art student in Six Feet Under. This spilled over into cinema.

As a sadistic cowboy in 3:10 to Yuma or an Iraq war veteran bringing bad news to soldiers’ families in The Messenger, a certain intensity was to be expected. But regardless of whether the work is highbrow (Kill Your Darlings) or low (X-Men: The Last Stand), he gives it his all. There’s no reason to expect anything different from him in the upcoming video game adaptation Warcraft. “I get to play a motherfucking wizard!” he exclaims. “Walking in what is ostensibly a dress, with a staff, a wig and press-on nails, presents different challenges to riding a bicycle. Especially when you’re trying to open a Coke.”

Facebook Twitter Pinterest Lance Armstrong in 2003/ Photograph: Patrick Gardin/AP

The director Oren Moverman worked with Foster on The Messenger and Rampart before forming the production company Third Mind Pictures with him. “I first became aware of Ben’s work in 3:10 to Yuma,” he tells me. “When he appeared on screen, my immediate reaction was: ‘Who in the world is that?’ He had an energy and ferociousness that immediately made me curious. When we were casting The Messenger, Ben had a meeting with our producer who called me right after to tell me I have to meet this guy. He said: ‘Look into his eyes. It’s all there. I’ve never met anyone like him.’ As soon as we sat across from each other, I knew this was not only an extraordinary man and an extraordinary actor, but a guy who will teach me about directing. He was the real deal. Still is.”

Ben Foster in X-Men. Photograph: Everett/REX Shutterstock

Though this remains true, Foster has observed in himself a change of attitude. He measures out his explanation like grains of sand: “I’m learning not to hold on so tightly to my solitude.” As a young man, he says, he got caught up in suffering for his art. “It’s not an economical way to work. A driver would call it ‘white-knuckling’. If you’re holding on to the wheel so tightly, it’s gonna lock up your driving. Releasing myself from trying to control everything has been part of growing up. If I saw someone treating me the way I treated myself in my twenties, I’d kick his ass!” No wonder he doesn’t get offered the comedy roles he professes to crave. “My heroes were always Looney Toons, Robin Williams, the Three Stooges. I think everything I do is kinda funny. I think I’m sort of ridiculous. I bothered Judd quite a bit after Freaks and Geeks: ‘Let me play with you guys again. Come on, I wanna laugh!’ He was, like: ‘Nah, you’re fine doing your serious work.’ What can I do?”

He puts his recent transformation down to age. “It’s really nice seeing the old-timers smile. I’d love to have more of that. I’m not going to beat this life. It’s gonna get me in the end. So accepting that is a real freedom, and finding a joy in working that I haven’t allowed myself before. I always thought you had to bang your head against a wall or stay up all night long. The work is much closer to the surface now. It’s easier to access and also to let go after a movie. It’s a gentler landing.”

That said, he sometimes finds old ghosts returning to him unexpectedly. In London last year to appear on stage opposite Gillian Anderson in A Streetcar Named Desire (a production that will be revived in New York next March), he found that William Burroughs, whom he played in Kill Your Darlings, kept surfacing in his thoughts. “I’m doing the laundry and suddenly there he is and I’m talking to myself in his voice.” In all likelihood, then, he could have Lance Armstrong popping up in a few years’ time while he’s pushing a supermarket trolley.

He narrows his eyes and wags an accusatory finger at me. “Talk about marbles. You’re pushing me in the direction of a complete multiple personality disorder here, aren’t you? That’s where we’re going, right? ‘Ladies and gentlemen, I give you … Crazypants Foster!’”

The Program has its world premiere on Sunday at the Toronto International Film Festival. It opens on 16 October.