Jake Gyllenhaal was ''terrified (he'd) look like an idiot'' playing a boxer in 'Southpaw'.

The 34-year-old actor is portraying light-heavyweight champ Billy Hope in the new boxing film, and has admitted he bulked up for the role, because he wanted to know how it felt to be hit, so he could play the part better.

Jake said: ''I didn't know how to box and I was just terrified that I'd look like an idiot in the ring. That was a motivation for me getting in shape.

''I don't think you can claim you're a boxer without knowing how it feels to be hit.''

The 'Donnie Darko' star spent five months in the gym to play his part in 'Southpaw', which involved two punishing three-hour workouts seven days a week and 2,000 sit-ups a day, and has admitted it meant he had no social life.

Speaking to the Telegraph newspaper, Jake said: ''The first part of the day was all technique, learning how to throw punches, footwork, and the second part of the day was getting into shape and getting ready for sparring.

''A physical transformation was key to making the character credible. I didn't spend much time doing anything else or socialising in any other way.''