Bradley Cooper describes the intense preparation for his role in the Oscar-nominated American Sniper

Cooper says the responsibility he felt in playing Chris Kyle, who died in 2013 with the legacy of being the most lethal sniper in United States military history, was 'massive'.

Bradley Cooper spent 15-hour days, five days a week at the gym and with a dialect coach to ensure that he looked and sounded like the real Chris Kyle. Warner Bros Pictures

Associated Press

There's a lot going on for Bradley Cooper. One of his biggest movies to date, Clint Eastwood's American Sniper, is at the top of the North American box office. For the film, in which Cooper serves as both actor and producer, he transforms himself – to critical acclaim – into the late Navy Seal marksman Chris Kyle.

"I think the fact that I had a chance to play Chris Kyle and Joseph Merrick [subject of the Broadway show The Elephant Man] in the same year is huge for me," says the 40-year-old Cooper.

It's certainly a season for cinematic portrayals of real-life characters: Martin Luther King Jr in Selma, the physicist Stephen Hawking in The Theory of Everything and the mathematician and codebreaker Alan Turing in The Imitation Game, to name a few. Cooper, who is up for a Best Actor Oscar for his role in American Sniper (his previous nominations were in 2013 for Silver Linings Playbook and in 2014 for American Hustle), says the responsibility he felt in playing Kyle, who died in 2013 with the legacy of being the most lethal sniper in United States military history, was "massive".

“There’s a family that’s still alive and children that are still alive,” says Cooper. “So this film, if we get it right, it will matter to them forever. For the rest of their lives and their children’s lives, potentially. So that was a huge responsibility.”

Cooper’s response to the challenge was “to put a tremendous amount of pressure on myself”. For starters, that meant “gain 40 pounds and literally learn exactly how he spoke. Wake up 5am, put on my headphones right away, listen to his voice right away, just to get it in my system,” he says, adding that he would then ride his “motorcycle to the gym, where I have big blow-up photos of him”.

Two-and-a-half hours of heavy lifting followed, staring at those photos and listening to Kyle’s playlist, given to him by the sniper’s widow, Taya. Then two hours at home with a dialect coach, back to the gym for two more hours, back home for more speech work, until 8pm. “I would do that five days a week,” he says. “Yeah, it was intense.”

Cooper also spent hours listening to every interview Kyle had done. He had no personal memories to go on, only a brief phone conversation. Kyle was shot and killed at a shooting range in February 2013 by a former Marine.

So the research was extensive. “What we really cared about was, first of all, the family believing that it was Chris up there,” says Cooper. “They were very, very positive in that direction. And that meant the world to me.”

Taya confirms her approval. “I really don’t know how he did it as well as he did,” she says. “So many of Chris’s friends said it’s almost eerie watching it. I’m eternally grateful.”

Cooper's Broadway run ends on February 22 and he hopes to take the show to London in the summer. As for film, whatever's next, he doesn't think it'll be easy to top American Sniper.

“I was just saying to my buddy this morning – ‘if we’re lucky, we’ll get to do this again,’” he says. “But it’ll never get better than this.”