He plays Dave Brown, a man who's been on the force for more than 20 years. The film takes place in the rough and tumble times of the LAPD's post-Rodney King scandal ridden era.
"The moment I read it I recognized two things: one, maybe one of the best scripts I ever read or maybe the best, and two its the best part I've ever been offered," Harrelson said.
The actor jumped at the chance to dive into Dave's dark world. His character is fictional; however, writer James Ellroy gathered his information from stories of real renegade cops, those who manipulate the system for their own gain.
"I suppose that every individual whether he's a cop or not is going to have his own sense of morality, sense of right and wrong. Some people think it's ok to lie. You know if you're a politician you're going to have to be lying just almost continuous, that's acceptable for them," he said. "As Dave Brown, he started to believe, 'Hey I'm doing bad things to bad people,' and that's the justification."
Harrelson also worked hard to find a human side to his character, and that meant showing the guy's family life.
"That's the core of Dave Brown and that's why he you know is not just entirely hate-able, you know there's some gray area with Dave Brown because he actually does love his family and you can see that," Harrelson said.
"Rampart" is rated R and is in limited release this weekend.