When it came time for Bradley Cooper to transform his body for director Clint Eastwood's "American Sniper," the actor says his past struggles with addiction dictated the workout regimen. 

"I did it naturally because I’ve been sober for 10 years and didn’t want to do anything," the 39-year-old told Vanity Fair. "I had a realistic conversation. Can I do this in three months naturally? Can I gain 30 pounds of ... muscle? I didn’t know if I would be able to do it or not. Thank God — luckily — my. .. body reacted fast."

Cooper plays Chris Kyle, an elite Navy SEAL sniper who holds the record for the most kills in American military history. Tragically, Kyle was killed in 2013 after an incident involving another war veteran at a Texas shooting range. From the beginning, Cooper was adamant about honoring his memory with as genuine a performance as possible. 

"I gained about 40 pounds," he said earlier this year. "I'm about 20 down from that now. I ate 5,000 calories a day and doing whatever I could to pay homage to [Kyle]." In addition, the actor also trained with Navy SEALs and learned how to use three different types of rifles. 

"I had to basically change my complete body chemistry," he told Deadline. "I had to learn how to talk like him. I had to be pretty efficient with two bolt-action sniper rifles and be able to clear houses with an M4 and carry all that weight — I had about 60 or 75 pounds’ worth of gear. To be able to do all that stuff without thinking about it took a tremendous amount of preparation, but that was a good thing, because by the time I showed up, I was ready to go."

Check out the trailer for "American Sniper," which hits theaters on Dec. 25, in the video below. 

Related on MNN:

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

Bradley Cooper's all-natural transformation for 'American Sniper'
Actor cites past struggles with addiction as a big factor in his decision to use absolutely no stimulants to gain 30 pounds of muscle.