Woody Harrelson tells Jeremy Kay how he prepared to play the policeman at the centre of Rampart.

In Rampart, Woody Harrelson stars as wounded lion Dave Brown, a corrupt police officer who prowls the 1999 Los Angeles hinterland in a nervy two-step with fate, ducking investigatory hearings and struggling to protect his unorthodox family.

“The key emotion running through Dave Brown is paranoia,” Harrelson says of his role in firm friend Oren Moverman’s follow-up to The Messenger, in which he also starred. “I invited that fucker in and it’s still wreaking havoc.”

Harrelson remains “a happy hippie from Hawaii”, as he puts it, but creating Brown was intense. “This was the best script I had ever read [Moverman co-wrote with James Ellroy],” the two-time Oscar nominee recalls. “And the best part I had ever been offered. There was also the sureness this would be a challenge.

“I could not even remotely see myself as a cop,” Harrelson says as a lazy grin rolls across his face. “It might have something to do with my authority issues.” Harrelson went on patrol with the LAPD and gradually everything clicked. “They were incredible — powerful, interesting, funny. It was about coming to respect and appreciate these guys and feel their humanity.”

To create the feral Brown, Harrelson lost 29 pounds, pushed his hair back and wore plumpers for his teeth. But the toughest transformational challenge was the accent. “I worked with a dialect coach to shed any trace of Southern, which I realised I rarely let go of.”

The shoot took place in Los Angeles in late 2010. “Oren never cuts. It’s pretty cool to work with someone where the whole tapestry of the scene is open to the moment.”

Harrelson saw an early cut in May and hated it, saying: “I thought Oren had lost his vision. I didn’t want to go to Toronto and do interviews.” A subsequent edit before the world premiere restored his faith. “I told him this and he started crying. He’s a big guy and I’d never seen him cry before and I started bawling too.”

Harrelson has The Hunger Games coming up for 2012 and the Sarah Palin HBO film Game Change. But the brotherly love will continue, with Moverman recently hinting at a new story. “He said three words, ‘Manhattan. Murder. Mystery.’” The grin returns. “I’m in.”