BEN FOSTER is adamant - there is one person without whom his new picture, The Program, would not exist. Would that be the film’s subject, the disgraced Tour de France champion Lance Armstrong? No. How about the director, Stephen Frears? Or the journalist David Walsh, on whose book the film is based? Important as they were, no.

Instead, glide forward Scotland’s David Millar, a sporting hero with his own chequered past when it comes to doping.

“David was the great conductor and hand to hold through all the cycling,” says Foster, whose dedication to looking and acting the part of Armstrong extended to taking part in a ferocious training regime. Controversially, Foster went one stage further and took performing enhancing drugs to see what it felt like. Millar, like the rest of the cast and crew, was unaware of what the actor was doing.

“He did not know that I was doping, and I think he would have discouraged me from doing that greatly,” says the star of The Messenger, Lone Survivor, and 3:10 to Yuma.

The Program, directed by Stephen Frears, tells the story of the Armstrong from his early wins, through this successful battle against testicular cancer, on to to his seven Tour triumphs, his comeback, and subsequent exposure as a drugs cheat. Before casting, Foster knew little of Armstrong other than the headlines. By the end of his research he had formed a nuanced view of a complex character.

“I don’t think it’s the popular point of view but I think he is a remarkable warrior,” says Foster. “I don’t necessarily like the way he did certain things but I really appreciate the will of a man in a time that is sullied.”

Having delved into that period, Foster believes Armstrong takes an undue share of the blame for what was a widespread activity. “He just did it better. He did it all better,” he says of Armstrong’s training, goal setting, endorsements, and presentation of his story. “Everybody was doping. To blame him solely is, I think, convenient.”

In the film, we see Foster’s shape change dramatically. From a normal, fit person he turns into a lean, mean, cycling machine. It must have been satisfying to attain that level of physical peak, I suggest. “It was a ******* nightmare,” he shouts, laughing.

Actors on the junket circuit are not supposed to say things like this. They are meant to tell journalists that the training was tough but they enjoyed it, all that blarney. Foster is not that kind of PR-speak merchant. Just as he goes all out to capture the characters he plays, so he tends not to hold back in interviews. All of which makes him a pleasure to meet. “I see a bicycle now I burst into tears,” he tells me.

It perhaps helps that Foster, born in Boston, brought up in Iowa and now living in New York, has just come back from a flying visit to Glasgow to see director David Mackenzie, with whom he is making a crime drama, Comancheria, and is feeling toasty towards the Dear Green Place and those who hail from it. “My stomach relaxed when I got there. I don’t know how to say it other than that. The way that people are with each other felt human. Good for a laugh, not putting on a pretence or a class. A little different.” When it comes to being chilled, it helps, too, that Foster is a lifelong devotee of mediation. It’s a way to “quiet the racket” he says.

Helping Foster through the pain of training was Millar, who fought his way back to a successful career after serving a two-year doping ban from 2004. “He is such an affable human,” says Foster of the Scot who became a fervent campaigner against doping. “And he is a very gentle teacher. I was 30lbs overweight, and whereas Stephen [Frears] would call me ‘Fatty Foster’, he would say, ‘Well, maybe just don’t eat the bread’. He was really a gentleman about it.”

The only person Foster consulted about taking performance enhancing drugs was a doctor. “It’s not something I’d recommend,” he says. And he soon felt the effects. It was like being 15 years old again, he says. “I could climb farther, I could go faster, I wouldn’t get tired or gas out. It just works.” But weren’t there side effects, jangling nerves, interrupted sleep?

“None of that. It was all great. I slept like a log. It’s when you stop taking the drugs, that’s where the deficit is.” Later, his body went into “a bit of a crash” and he felt rough for a while. “You gain weight and cholesterol. The side effects are real. However, knock wood… there was a short fall out, and for my own investigatory interests I’d do it again.”

Even before The Program, Foster had a reputation for being “method” and going above and beyond to capture a character’s experiences. He thinks it is simply doing his job. The vehemence with which he does it has earned him inevitable comparisons with that other tower of intensity, Marlon Brando. The connection goes further in that Foster recently played Stanley Kowalski in A Streetcar Named Desire on the London stage. The production is due to transfer to Broadway next year. “I don’t think you can talk about Kowalski without talking about Marlon. Marlon was like Picasso for acting. He shattered it. He broke it apart.”

There is a forthcoming documentary about Brando called Listen to Me Marlon. It is built around recordings that the actor made for himself in which he talks about his life and art. It is fascinating stuff, full of revelation, including that he hated Kowalski as a character. There, Foster and Brando go their separate ways. “I don’t know how to hate a character and commit to it. I don’t know how to take that job on.“

Foster wonders if Brando was trying to be provocative. “He’s always messing with the system and himself. What strikes me about his work is his profound empathy, whether or not he says he hated Kowalski. I love Stanley.”

And he will love him again on Broadway. Also coming up is Inferno, the adaptation of the Dan Brown blockbuster. The Program finds Foster mid way, once more, between small budget indies and mega releases. Could he get used to the blockbuster lifestyle? It doesn’t seem so.

“The movie I could make with the money that’s spent on the hotel rooms for film junkets makes my skin crawl. But I don’t control that. And I’m deeply grateful.”

In one sense, we agree, the biggest mistake Armstrong made was coming back for it was then that he was caught. I wonder if Foster could leave the film business and not look back.

“I’ve made hollow threats once every two years to my friends and family, like, I’m done with this nonsense, I’m a full grown man, I shouldn’t be wearing someone else’s clothes, saying other things, but the truth of the matter is ….” he trails off.

The truth of the matter is that Foster’s career, having been going at quite the lick till now, is about to move into top gear.

The Program opens in cinemas tomorrow