Damon was determined to slim down for his role in the wartime drama, and despite cutting out fatty foods he was baffled as to why he appeared to be gaining weight - until his co-star and director Clooney confessed he had been playing a practical joke for weeks.

Clooney tells U.K. talk show host Graham Norton, "When we were filming The Monuments Men, Matt was trying to lose a few pounds and was only eating salad and was really serious about losing weight. I had the wardrobe girl take his costume in by just a quarter of an inch each day. He was then just eating a grape and saying, 'I don't understand why I'm getting fatter.' I did that for about three weeks!"