De Niro famously packed on 60 pounds for "Raging Bull."
"I talked to Robert De Niro before I did it," Damon said at a recent Toronto International Film Festival news conference. "And he said, 'Well, the first 15 pounds are really fun and then you have to go to work after that.' And it was true, almost, because I found 30 pounds really fun and then, towards the end, I was like, 'I should get rid of this weight.' But I wasn't really that excited about getting rid of it."
Asked by a journalist how he got in touch with De Niro, Damon reminded her he had collaborated with the Oscar-winning actor on the movie "The Good Shepherd" a few years back.
"We worked together. I didn't cold call him," Damon quipped. He then pretended to make the call, saying: "Hi, is this Robert De Niro? Listen, I'm a young actor and I need some advice. Got a minute? Have you ever seen 'Mystic Pizza?' I'm in that, Ok? How about 'School Ties?'"
Damon has also starred in the "Ocean's 11" and "The Bourne Identity" film franchises.
"The Informant!" opened in U.S. theaters Friday.