Foster revealed last week (beg07Sep15) that he followed Armstrong's doping programme under the supervision of a doctor to better understand the role, and spent many hours cycling on parts of the Tour de France route.
The actor did not tell his co-stars or director Stephen Frears about his drug use until days before the film's premiere at the Toronto International Film Festival last week.
O'Dowd plays Irish journalist David Walsh, who spent the best part of 13 years uncovering the cyclist's use of steroids, and he told the BBC that Foster's intense role preparation "made sense".
He says, "I think that's a really smart thing and makes sense. With something like that you know it's not going to harm you necessarily, and you're going to have to perform so much (while filming)...
"I think if you have to take drugs in a role, it's very hard to do without taking the drug at some stage in your life. I'm not advocating that you (should take drugs for a role), but I'm sure it probably wouldn't hurt your performance.
"It might hurt you as a human being, but it would be silly to say that would not be a good way in to knowing (what it's like). That's not to say you should, or you shouldn't, but you can understand the validity of it."
The Program is due for release in October (15).