How Bradley Cooper Got Really Ripped to Play a Navy SEAL in American Sniper

Bradley Cooper worked hard to get in shape to play real-life Navy SEAL Chris Kyle in the Clint Eastwood-directed American Sniper. PEOPLE caught up with writer-producer Jason Hall for the inside scoop on how Cooper trained to become the most lethal sniper in U.S. military history.

In order to bulk up 40-plus lbs., Cooper "had a strict, crazy diet," Hall says. "He was eating about every 55 minutes or something like that, and I want to say it was about 8,000 calories a day."

"Bradley used his own trainer, who was busting on him. I think he was working out four hours a day for several months," adds Hall of Cooper's fitness regimen. "He was determined to do it naturally, he didn't want to use any hormones or steroids or anything. He was just very systematic about it and took his trainer with him wherever he went."