The Massachusetts-born actor underwent an extreme physical transformation for a role in the 1996 film Courage Under Fire. Damon had to lose 60lbs in a short period of time to play an army medic in the war drama, and in a Reddit "Ask Me Anything" he revealed exactly how he cut the weight so quickly—and trust us, it doesn't sound fun.

Damon, who normally weighs around 190lbs, got down to roughly 139lbs to shoot the film, according to the AMA. "I had to run about 13 miles a day, which wasn't even the hard part. The hard part was the diet," he revealed. "All I ate was chicken breast. It's not like I had a chef or anything, I just made it up and did what I thought I had to do. I just made it up and that was incredibly challenging."

But while it may have been challenging, it was definitely effective!