Matt Damon: Courage Under Fire and The Informant!
Matt Damon has swung both ways: For the 1996 movie Courage Under Fire, Matt lost 40 pounds in three months by running 10-plus miles every day and eating only chicken, egg whites, steamed broccoli, and plain baked potatoes. He succeeded in looking like a heroin-addicted Gulf War vet (the point of the madness), but not without creating health issues for himself.
According to Starpulse, Matt later said, "I went too far. I got sick and I wouldn't do that again because it was just too much. At the same time it helped the performance. I didn't have to act at all — I was a wreck. I was getting dizzy spells and hot flashes. I didn't say anything to anyone for a while because I was afraid I might be really ill."
Twelve years later, the actor let it all go, adding 30 pounds to his typically 155-pound frame to play a pudgy corporate exec in The Informant! He dished to the New York Daily News, "At my age, all I had to do to gain weight was eat the way I did when I was in college and the weight went on instantly. It was a little horrifying."
He continued, “I ate pizzas, In-N-Out burgers and drank dark beers. [...] With me, the first 20 pounds were fun and then it started to really wear on me because I couldn't exercise, so I never got the endorphin rush of breaking a sweat.”