Cooper thanks war vets

Bradley Cooper viewed beefing up his physique for American Sniper as "a science experiment".

The 39-year-old actor stars as Chris Kyle in the new film. Chris was a Navy SEAL sniper who served in Iraq and then struggled to get used to home life again and the movie is based on his autobiography.

Bradley had to pile on muscle for the part, eventually gaining 30lbs.

"It was basically a science experiment, trying to get as big as that man was," he laughed to Variety. "There were a lot of vets that were part of the movie. There’s a lot of people who helped out to make it as authentic as possible."

The film was directed by Clint Eastwood, who was adamant about making it as true to life as possible. That's why so many servicemen were called in to help, among them Kevin Lacz. He is a real-life friend of Chris' and plays a Navy SEAL in the flick.

"I’ve been method acting my entire life,” he laughed. “I was training Bradley, and he asked me, ‘Have you ever thought about playing you in a movie'’ and I said, ‘No man, just shoot straight and get this right.'"

Eventually he did decide to get involved, with Bradley returning the favour by offering him acting tips as needed.

Bradley's training regime was intense for the film and he's spoken about changing his shape before. The star has battled addiction in the past, meaning he was certain that he couldn't use anything to artificially bulk himself up.

"I did it naturally because I’ve been sober for ten years and didn’t want to do anything. I had a realistic conversation. Can I do this in three months naturally' Can I gain 30lbs. 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," he previously explained to Vanity Fair.