The Irish star plays real-life journalist David Walsh in the new movie, which examines the disgraced cyclist's years of drug cheating during his Tour de France victories and the reporter's dogged quest to unmask him.

The project was considered top secret when director Stephen Frears was sounding out potential actors for the film, and when O'Dowd was called to the meeting he feared he was about to be offered the lead role, which would have meant a massive fitness drive to prepare himself.

"(Frears) said, 'I can't tell you what the project's about, but it's kind of controversial!'," O'Dowd tells British newspaper The Times. "He then said, 'Chris, what do you know about Lance Armstrong?' And immediately your mind goes a mile a minute, and I'm thinking, 'Could I play Lance Armstrong? Do I still have my bike? I'll have to lose a couple of stone!'

"And then he said, 'And what do you know about the Irish journalist David Walsh?' And I go, 'Ahhhh. That makes much more sense'."

O'Dowd admits he had moral doubts over whether to accept the part but decided to go ahead when he reasoned that Armstrong's cheating had undermined the sport.

"I have to say that when I came on board and asked myself, 'Do I care that he took drugs?' the answer was, 'Not hugely'," he adds. "But do I care that the sport is unfair because of it? Now, I care a bit more. And finally, 'Do I care that he made it entirely impossible to compete without taking drugs?' Well, that's it. That's the endgame for me."

The actor also reveals he was so determined to portray Walsh accurately that he kept away from co-star Ben Foster - who plays Armstrong - for the entire shoot in case they became pals.

"Honestly, we never hung out," O'Dowd says. "We were civil to each other. But I think there was an understanding between us that being pals was not going to be beneficial to this particular project."

Armstrong was stripped of his seven Tour de France titles in 2012 when he finally admitting doping during his race career after years of denying the rumours.