Damon estimates that he gained between 25 and 30lbs in preparation for the movie 'The Informant,' directed by Steven Soderbergh.
"It wasn't necessarily that I needed to be fat," he said. "It was that I needed to be doughy."
Even though the weight-gain was required for the role, that didn't stop Damon's friends from mocking him mercilessly.
"Some things are just self-evident and don't even require making fun of," his best friend Ben Affleck said. "I mean the man buys two seats on an airplane!"
So how did he manage to gain so much weight? "I just stopped working out and basically just ate whatever I wanted," he said. "I ate a lot of In-N-Out, a lot of burgers, a lot of beer and basically had a great time ... When you're in your 20s you can do that kind of stuff. When you're in your 30s, its a whole different ballgame."