"I did it naturally because I’ve been sober for 10 years and didn’t want to do anything. I had a realistic conversation. Can I do this in three months naturally? Can I gain 30 pounds of muscle? I didn’t know if I would be able to do it or not", revealed Bradley who has not touched alcohol since he was 29.
He added: "Thank God—luckily—my body reacted fast."
American Sniper is based on the true story of Chris Kyle, a United States Navy SEAL, who claimed to have 160 confirmed kills and became known as America's deadliest sniper.
American Sniper poster
During the interview with Vanity Fair, Bradley also discussed how losing his father was a turning point for him, admitting: "Realising that the bottom line is that all I got is me, so it’s about time to stop trying to be something that I think you would want me to be. Or that would give me what I think I need.
"As you get older, thank God, your body deteriorates, but your soul sort of flourishes."
He added: "I see life much more grey as I get older.
Bradley takes the curtain call at The Elephant Man
"I was so sort of black-and-white in my late 20s. There’s right and there’s wrong and that’s it. That’s a tough way to live. It’s rare that I judge somebody, really rare. I think people feel that, so it’s sort of easy to get close to somebody if you don’t feel judged by them."
Bradley recently received rave reviews for his return to Broadway for The Elephant Man, but revealed that his debut on stage in 2006 with Julia Roberts and Paul Rudd in Three Days of Rain, was his last chance.
"If this doesn’t work, then I’m not supposed to do this for a living," he told Vanity Fair of the play.