The 38 year old stars as bipolar whistleblower Mark Whitacre in dark comedy thriller The Informant!, based on the 2000 book of the same name.
Damon had to gain 30 pounds (13.6 kilograms) for the part - and he happily indulged in fat-packed fast food and alcohol to bulk up.
He tells the New York Daily News, "I definitely got doughy. I started eating like crazy and drinking dark beer. Between meals on set, I'd eat a No. 1 Value Meal at McDonald's and then Doritos on top of it. It was absolute heaven.".