Bradley Cooper has revealed his gruelling preparation for American Sniper.
The Hangover actor is playing former Navy SEAL Chris Kyle in the big-screen adaptation of his memoirs, which Clint Eastwood is directing.
"The workout regime was pretty brutal. At one point I was working out so intensely that I was eating 5,000 calories a day. I gained around 40 pounds," he told Total Film.
Bradley also worked with real US army men to get first-hand experience.
"I wanted to do this story justice and that was never going to happen unless you do all the preparation you can. I trained with the Navy SEALs, and they were actually the same SEALs that worked directly with Chris," he explained.
"I trained with Kevin, who was Chris' sniper partner, and we had him as technical advisor on set because we wanted to get everything right. I was just acting but when I was looking through the (sniper) scope on set, I got this cold feeling that just ran through my body. You don't realise what these guys go through: we wanted to delve into that."
Bradley got to speak with Chris before he was killed in 2013.
"I spoke to Chris before he was killed just once. But thank God I got the chance to at least make that one call. It wasn't a long conversation," he recalled.
"I told him whatever fears he had about Hollywood that he didn't need to worry because I wanted to get the story right. After Chris Kyle's passing, I knew we had a duty to get things right for his memory and for his family, and I am confident we have done that."
Sienna Miller and Luke Grimes also star in American Sniper, which will open in UK cinemas in January 2015.