I think this is a great article where Jim talks about his experience while shooting Fifty Dead Men Walking.
Below is an excerpt of the article.
Sturgess was the opposite. Whereas Kingsley’s character is more cerebral, more of a coaxer, Sturgess plays the doer, a young man caught in a violent mess. The actor hung out with Belfast residents in the weeks before shooting began. Local guides introduced him around pubs and house parties. It was also the first time that Sturgess, a native Londoner, maintained the accent of a character for the entire production, even when talking on the phone with his mother, he says.
“You live in the accent, and by doing that it changes you as a person. If you change the way you speak, it changes the way you behave,” he says. “I remember I became a lot more cheeky, because the accent sort of allows you to have that cheekiness in your personality.”
He adds that he never met Martin McGartland, the informant whom Sturgess’s character is roughly based on, and who wrote a 1997 memoir with the same name as the film. There was enough material in the book and the script to understand his role, Sturgess says, without having to try to adopt McGartland’s specific mannerisms. Instead, the actor relied on the city and its stories.
“It just wasn’t black and white, there was so much grey area in between. It was about filling in those grey areas and finding out what they were for everybody,” Sturgess says. When the actors and crew shot on location, local residents gathered to watch. What was their reaction given the tensions undoubtedly still raw for some?
“They were excited, they kind of loved it. Kids would hang around and steal our sandwiches. They were just such a good group of people. Literally, we would be filming on the streets where [the violence] really happened. And in the riot scene, a lot of the people in the scene were people who were there when it really happened to them all those years ago. And they were coming back, and they were reliving that experience,” Sturgess says.
Sometimes locals were asked by the film crew if Sturgess could quickly change outfits in their homes between takes. He invariably would be welcomed in and offered tea and biscuits. “The sense of community is so strong in those areas,” Sturgess says, “which I think helped me to understand the guilt – or the difficulties – that Martin McGartland would have gone through when he acted against some of the people of the community that were strong IRA believers or sympathizers.”
Source
Crossing Over
The Way Back
Heartless
