Bradley Cooper’s latest big-screen venture sees him take on his most challenging role to date in Clint Eastwood's upcoming biopic, 'American Sniper,' portraying Navy SEAL sniper Chris Kyle, who has more than 150 confirmed kills in his military career.

Bradley Cooper
Cooper portraying Navy SEAL sniper Chris Kyle in 'American Sniper'

However, portraying a real life war hero was something the Hollywood star though he wasn’t capable of doing, and initially thought another actor was much more suited for the role, his ‘Guardians of the Galaxy’ co-star Chris Pratt.

"Initially I thought, 'There's no way I could play Chris,'" he recently told CBS News. "I thought Chris Pratt should do it."

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Cooper, who gained 30 pounds of muscle in three months for the role, didn’t think he could play a Navy Seal and is "a buck 85."

"And then I started to get to know him," he added. "And there was something about his energy in these interviews that I started watching where I just sort of- there's something about him. And I thought, 'Oh, I can tap into that.'"

The project became even more sensitive when Kyle, 38, was tragically killed by a troubled veteran during an outing to a shooting range in Texas on February 2, 2013. However, Cooper was very dedicated to playing the SEAL, so he therefore worked closely with Kyle's widow, Taya Kyle, when preparing for the role.

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After creating such a strong bond with the Kyle family, Cooper recently told NPR, he "honestly can't imagine not being in the family's life. But that's up to them. The way I live my life, I'm very good at keeping in touch."