In the movie, Kyle (played by Cooper) recounts his military career, which involved 150 confirmed kills. Kyle eventually took an honorary discharge after four tours of duty in Iraq so that he could focus on his wife and family.
Tragically, Kyle died at a shooting range last year before the movie was made. Cooper only got to speak to Kyle once on the phone before he passed, and his death made Cooper even more determined to do the film justice.
"It was brutal," said Cooper, also a producer on the film, who gained 40 pounds to prepare for the role. "It was 5,000 calories a day." But the physical transformation was worth it, he said. "Just doing whatever I could to pay homage to Chris Kyle."
Watch the video for an intense clip from the movie.
Directed by Clint Eastwood, American Sniper opens January 16, 2015.