Like many top actors, The Social Network star likes to stay fit by strapping on gloves and hitting the bag at his local boxing gym, and when fellow member Shawne Merriman offered to spar with him in the ring, he foolishly agreed - and quickly discovered how the retired San Diego Chargers linebacker picked up his fearful moniker.

Hammer says, "This dude walks in and he's enormous and, as a joke, he kinda goes, 'Hey, you wanna spar?' and I was like, 'Yeah, sure man, throw on some gloves, let's do it', and he starts putting on gloves and I immediately realised I'd made a huge mistake.

"I come to find out his name is Shawne Merriman; he's a football player. I didn't know this at the time... I know very little about sports... so I just thought he was a big guy and you think, 'Oh, he's big, he probably doesn't move very quickly'; he moves really quickly."

Hammer admits he and his heavyweight opponent were "having fun" in the ring until he "caught him with a left hook".

Appearing on U.S. chat show Conan on Wednesday (05Aug15), he recalls, "I felt it connect... and he completely changed, like, his eyes went cold and he starts swinging at me... and I was like, 'I'm gonna die!'

"Two days later, the whole side of my face was swollen. It was ridiculous."