Matt Damon's character in Steven Soderbergh's new film 'The Informant!' didn't just have him wearing a fake mustache, hairpiece and glasses -- it also required him to pack on the pounds!

Damon -- the former People magazine's Sexiest Man Alive -- gained 30 pounds to play the role of Mark Whitacre, an agricultural company's vice president who becomes a FBI informant.

"It was very, very easy to gain the weight," Damon told reporters at the Venice Film Festival, according to People.com. "It was very, very fun, probably the funnest time I had working because I didn't have to go to the gym after work and I just ate everything I could see."

Check out Damon's "fun" weight gain when 'The Informant' hits theaters September 18.