Gerard Butler found it a "big advantage" playing a real life character in 'Machine Gun Preacher'.

The '300' actor got plenty of advice from former drug-dealer Sam Childers before he acted out his life in the movie, which tells the story of how the ex-biker found God and became a crusader for hundreds of Sudanese children who are forced to become soldiers.

Gerard said: "The challenge is there's someone you have to answer to who is at the end of the movie going, 'Hey, that was me or that wasn't me and you did too much of this or you did too much of that.'

"But more than anything it's a big advantage because you get to see that person and you get to see what goes on in a person's eyes when they talk about their life and get a lot of tips from them.

"It's not like everyone in the world knows who Sam Childers is. I wasn't impersonating Hitler or JFK so it wasn't an exact mimicry we were looking for."

The 41-year-old hunk - who starred alongside Michelle Monaghan in the film - thought the movie's director Marc Forster was "surprisingly fun", and he praised him for his conscientious work during filming.

He added: "Marc's surprisingly fun, when you meet him he's very intense and sincere. He's Swiss and they're like that. They say everything they mean. He really laid it all out and then when he opens up he is just one of the most humble, beautiful souls and yet he's so smart.

"It would be easy to be bowled over by this story of a hero and he was like, 'No, no, no. Lets examine the other side of Sam as well.' "