How Bradley Cooper Got Really Ripped to Play a Navy SEAL in American Sniper

Bradley Cooper in American Sniper

Courtesy Warner Bros.

By Aurelie Corinthios

11/13/2014 AT 12:15 PM EST

Just call him the Sexiest Sniper Alive.

Bradley Cooper worked hard to get in shape to play real-life Navy SEAL Chris Kyle in the Clint Eastwood-directed American Sniper . PEOPLE caught up with writer-producer Jason Hall for the inside scoop on how Cooper trained to become the most lethal sniper in U.S. military history.

In order to bulk up 40-plus lbs., Cooper "had a strict, crazy diet," Hall says. "He was eating about every 55 minutes or something like that, and I want to say it was about 8,000 calories a day."

"Bradley used his own trainer, who was busting on him. I think he was working out four hours a day for several months," adds Hall of Cooper's fitness regimen. "He was determined to do it naturally, he didn't want to use any hormones or steroids or anything. He was just very systematic about it and took his trainer with him wherever he went."

Bradley Cooper on the set of American Sniper

GC Images

Along with the massive intake of food and intense workouts, Hall revealed that Cooper's preparation also included twice-daily lessons with a vocal coach, as well as several hours spent watching footage of Kyle, whom he studied "over and over again."

When it came to actually pointing a rifle, the actor trained with Navy SEAL sniper Kevin Lacz, who served with Kyle (who was killed by a fellow veteran in 2013) and was a consultant on the movie.

And in case you were wondering, Cooper's sniper skills are on point – pun intended.

"He actually proved himself to be really good," Hall says. "The second day, in the morning, he went out there and was consistently hitting 800-yard targets the size of a teacup. So he took to it pretty quickly."

RELATED: Watch Bradley Cooper's American Sniper Trailer

But the real feat proved to be the way Cooper brought Kyle "back to life," Hall says.

"He managed to find this essence of Chris Kyle," Hall explains. "This almost spiritual mentality of the man and what drove him. There were moments where they'd get him at a certain angle and you just felt Chris, you felt the essence of Chris. Standing behind the monitors, I'd just get goose bumps rushing up my legs."

American Sniper comes out Dec. 25.

• Reporting by JEFF NELSON

For more exclusive dish on the holiday season's biggest movies, pick up the new issue of PEOPLE, on newsstands Friday