Bradley Cooper felt the weight of “massive responsibility” during filming of American Sniper.

The 39-year-old actor has received critical acclaim in the form of Oscar nominations for his work in Silver Linings Playbook and American Hustle. And he was equally dedicated to stacking on pounds to portray a deceased American Navy SEAL in the new Clint Eastwood movie.

"American Sniper was unique because it became a massive responsibility to honour [real-life Navy SEAL] Chris Kyle after he [was killed] last year,” he told The Hollywood Reporter.

The biographical film tells the story of one of the most lethal snipers in the American military. He earns respect and hero status, but comes home to face emotional demons.

“I talked to him when we bought the rights to his memoir, then everything changed,” Bradley explained. “That responsibility allowed me to have endless energy to prepare.”

It wasn’t just an emotional task, however. It was also a physical one, with the actor required to alter his own state in order to achieve a likeness to Chris.

“One way was to get as big as he was. I was 185lbs then gained 40 because he was a big mother***er,” the star went on. “We were the same height and frame and shoe size. He was just big. He grew up in Odessa, Texas, and his accent changed according to who he was with. I had to get the size and the voice in order for myself to believe him. People in Texas said you have to have a lot of balls to play Chris. The surprise was that I always felt protected by him, like I was telling the story with him."

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