Clint Eastwood’sAmerican Sniper,” based on the life of decorated Navy SEAL Chris Kyle, premiered in New York Monday at the Jazz at Lincoln Center’s Frederick P. Rose Hall.

The film, adapted from Kyle’s autobiography “American Sniper: The Autobiography of the Most Lethal Sniper in U.S. Military History,” tells the story of how Kyle went from a traveling cowboy to one of the most lethal snipers in American combat history, and how he transitioned from his time serving in the Iraq War back into civilian life. Many of the most memorable scenes demonstrate how everyday civilian noises can put soldiers on edge.

Bradley Cooper spoke with Variety about how he trained with the Navy SEALs and worked with a nutritionist to gain enough muscle to resemble Kyle. “It was basically a science experiment, trying to get as big as that man was,” said Cooper, who credited the various servicemen that Eastwood brought in with helping to maintain verisimilitude. “There were a lot of vets that were part of the movie. There’s a lot of people who helped out to make it as authentic as possible.”

One of those people is Kevin Lacz, a SEAL, technical adviser and friend of Kyle’s, who has a noteworthy role in “American Sniper” as the SEAL Dauber. “I’ve been method acting my entire life,” he joked. “I was training Bradley, and he asked me, ‘Have you ever thought about playing you in a movie?’ and I said, ‘No man, just shoot straight and get this right.'”

Eventually, Bradley convinced him to act, and served as his acting coach, along with Eastwood. “It’s great to be a part of it, and it’s awesome to see these parts of history, Chris’s story put up on the big screen for everybody to see,” Lacz said. After Kyle was murdered in 2013, Lacz invited screenwriter Jason Hall down to Texas to meet Kyle’s widow (who attended the premiere), as well as his friends and fellow soldiers, so he could get a sense of the man frequently referred to in the film as “a legend.”

“Chris was a very down-to-earth guy,” Lacz explained. “If he was signing books, he would stand up the entire time, because there were people around the block, standing up, waiting to talk with him. I think that’s what set him apart; he’s a very humble guy, and one of the most accomplished snipers out there.”

“American Sniper,” from Warner Bros., opens in theaters nationwide on Jan. 16.

(Pictured: Sienna Miller, Clint Eastwood and Bradley Cooper at the “American Sniper” premiere)