Bradley Cooper refused to use stimulants to help him bulk up for 'American Sniper'.

The 39-year-old actor, who has been sober since August 2004 after struggling with drugs and alcohol addiction, was forced to gain 30 pounds of muscle in a short space of time to play a Navy SEAL in the film and feared he wouldn't be able to transform his body by simply working out.

He said: ''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 f**king muscle? I didn't know if I would be able to do it or not. Thank God--luckily--my f**king body reacted fast.''

Bradley also revealed the death of his father, Charles Cooper, in 2011 helped him prepare for the gruelling role.

He told the January issue of Vanity Fair magazine: ''Losing someone close to me. Going through love and loss . . . knowing what's important. 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.''

The 'Silver Linings Playbook' star, who has been in a relationship with British model Suki Waterhouse, 22, for almost two years, says his personal struggles have changed his approach to life.

He said: ''I see life much more gray as I get older. 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.''