Cooper is getting tons of praise for his performance as real-life Navy SEAL Chris Kyle in Clint Eastwood's American Sniper
, and now the film's writer-producer Jason Hall is dishing on the hard work the 40-year-old actor put in to embody his controversial character
Cooper had to gain 40-plus lbs. for the role, which meant going on a "strict, crazy diet."
"He was eating about every 55 minutes or something like that, and I want to say it was about 8,000 calories a day," Hall reveals to People
And of course, working out intensely was another necessity.
"Bradley used his own trainer, who was busting on him," Hall adds. "I think he was working out four hours a day for several months. 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."
Another notable part of Cooper's performance is the accent he uses, which he achieved through twice-daily lessons with a vocal coach, as well as hours and hours spent watching footage of Kyle.
But clearly, all the hard work has paid off. Aside from nabbing six Oscar nominations including one for Best Picture, the film is a true box office success, raking in more than $100 million
over the holiday weekend.
"He managed to find this essence of Chris Kyle," Hall says about Cooper's performance. "This almost spiritual mentality of the man and what drove him. There were moments where they'd get him at a certain angle and you just felt Chris, you felt the essence of Chris. Standing behind the monitors, I'd just get goose bumps rushing up my legs."
"It was pretty brutal," he admitted.