Jamie Foxx and Gerard Butler team up for a deadly game of cat and mouse in the psychological thriller 'Law Abiding Citizen,' and ET has a first look with the stars on location at the Philly set!

"There's lot of bang bang boogie in this movie," Jamie tells ET. "You know what I'm saying? Explosions. The way the movie opens is gripping, so it really has that tone of a 'Seven,' but only this time you get a chance to see [the crimes being committed]."

In theaters in March 2010, 'Law Abiding Citizen' finds Gerard as a man whose wife and daughter are brutally murdered. Looking for vengeance, he focuses his rage against Jamie's character, an assistant district attorney who allowed mercy to supersede justice in the case against the killers.

"It's about this regular guy who, when he turns his mind to it, can really bring a whole system to its knees," says Gerard, who reveals that he was originally supposed to play Jamie's D.A. character. "There was just something about this role that I found very tantalizing. ... It was something very interesting to play and to think about, and I thought I don't get that chance very often, so we did a swap."

"Gerard's character suffers a loss with his wife and his child; I'm on the opposite end of it," explains Jamie. "I'm really about moving up on the ladder, going from a D.A. to hopefully mayor of the city. Insensitive to his needs at the time … ten years later he comes back with all the wrong answers. He wants to get everyone back, and that's not the way it should go."

Despite such serious subject matter, Jamie says he definitely likes to keep the mood light on the set: "Oh, you know me man -- I'm always about the jokes," he says. "We just always have fun. We talk about the cold a lot. We talk about how Philly can get so cold. The sun is shining, but it is only 22 degrees. So we always get a kick out of that."

"Jamie's a funny guy; I temper it, you know?" says Gerard, who adds that Jamie loves to sing to everybody in between takes. "I like to try my best, but most people tell me I'm not funny at all. But in this, I think it's important to choose your moments for when to get heavy. If I have a heavy day, then I'll try to stay away from everybody."

Viola Davis, Bruce McGill, Colm Meaney and Regina Hall also star in the film directed by F. Gary Gray ('The Italian Job').

"We're shooting a lot of very intense scenes, and in between takes there's a lot of laughter," says Gary. "You would expect it with Jamie, but definitely with Gerard too. He's really, really funny."

Watch ET for more behind the scenes action from 'Law Abiding Citizen'!