But Cooper -- who packed on 40 pounds of muscle for the role -- says it isn't a film about battlefield action; it's about people.
"I never saw it as a war movie, I saw it as a character study," Cooper told Suzanne Marques of CBS Los Angeles. "So, I never even contemplated anything other than someone would get to know this man and his family."
Sienna Miller plays Kyle's wife, Taya. The actress says the role gave her a greater appreciation for the families of those on the front lines.
"The people who are the spouses of military servicemen and women are sacrificing a huge amount as well, and it was a really interesting thing to explore."
Also among the cast, three U.S. service members. Cooper says they helped to add authenticity to the story. But they also instilled a sense of pride among the cast.
"Everybody felt very -- you could just feel it. Everybody felt like it was a privilege every day to be on set telling this story, Cooper said.
"American Sniper" was directed by Clint Eastwood and has already been honored by the American Film Institute, where it won film of the year, and the National Board of Review, which awarded Clint Eastwood with best director.
Soren Anderson of the Seattle Times said the film, "is arguably his best picture since 'Unforgiven.'" Richard Corliss with Time magazine called it a "gritty, confident portrait of a man whose life may have been somewhat messier than this Hollywood version."
"American Sniper" opens nationwide on Friday.