Damon Had 'Funnest Time' As A Fatty

The usually hunky star packed on 30 pounds (13.6 kilograms) to portray bipolar whistleblower Mark Whitacre in the dark comedy.

He ploughed his way through fast food MCDonald's and snacks in preparation for the role - and added to his unflattering look with a handlebar moustache, a wig and thick glasses.

But looking less than handsome didn't get the 38 year old down - he loved having an excuse to become a slob and quit working out for several months.

He tells People.com, "It was very, very easy to gain the weight.

"It was very, very fun, probably the funnest (sic) time I had working because I didn't have to go to the gym after work and I just ate everything I could see."