"It was really hard because I love to eat and drink!" Damon, 42, told PEOPLE on Tuesday at a special New York screening of the sci fi film hosted by 2(X)IST Watches. "It's very hard to motivate me to get in shape."
But when the film's director Neill Blomkamp showed him pictures of Elysium's leading character, Max, a tattoo-covered tough guy who wears an exoskeleton strength suit, "I went okay, this is definitely worth it," Damon says.
The recent Emmy nominee went on a strict diet and spent four hours a day at the gym for months, pumping iron and doing cardio. As a result, PEOPLE's 2007 Sexiest Man Alive bulked up with a chiseled set of rock-hard abs and bulging biceps.
So what did Damon's wife Luciana think of his ripped physique?
"I think she likes the variety," he says with a laugh. "When I did The Informant, I put on 40 lbs. and I was like, 'You kind of like this guy too?' I'm trying to talk her into [liking] that guy because that's the guy I'm most comfortable being."
Fortunately for the Oscar winner, who renewed his wedding vows to Luciana in St. Lucia last April, she made a promise to spend the rest of her life with him no matter what.
"I'm at the point in my life where somebody was silly enough to marry me," Damon says, "and won't leave me if I put on a few pounds!"