Foster transformed his entire physique to play Lance Armstrong in Stephen Frears' new biopic The Program. And Foster admits that he didn't know much about the cyclist beforehand. "I knew he was the greatest at one point, and I knew he was considered a liar," Foster says. "but I had no preconceptions. 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."

The Program was initially titled The Cycling Project

Foster was determined to tell Armstrong's story as accurately as possible, without judgement. "He's a smart man," Foster says. "He said, 'I can do some good with this,' and 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 s**t. And we're punishing him because he didn't apologise in the way we'd like."

In addition to training with a professional cycling team in Colorado, Foster has admitted to experimenting with performance-enhancing drugs to better understand Armstrong. "Even discussing it feels tricky, because it isn't something I'd recommend to fellow actors," Foster cautions. "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. You just don't gas out when ordinarily you would feel tired. And if you push through that you have big reserves. It's night and day. The worst part is coming off it, because you're tired, your bones hurt, the recovery is longer. It's like ageing 10 years."

Foster isn't quite sure if it was the drugs or the intensity of getting under Armstrong's skin that left him feeling worn out after production ended. "I've only just recovered physically," he says. "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."

Watch the trailer for The Program: