Actor Woody Harrelson confided that he had difficulty portraying a cop in Rampart, which premiered at the Toronto film festival, but critics praised his performance as his best ever.

Dozens of actors have given memorable performances as corrupt cops over the years, including Harvey Keitel in Bad Lieutenant (1992), Gary Oldman in The Professional (1994) and Richard Gere in Internal Affairs (1990).

Denzel Washington won an Oscar for his performance as a dirty cop in Training Day in 2001.

“I didn’t try to stack myself up against Harvey Keitel or any of those other performances because if I were to think that way I’d have shot myself in the foot before I got out of the gate,” Mr Harrelson told a press conference, “because those were amazing performances”.

“To me it was about coming to believe that I could be a cop. That was my hardest thing,” Mr Harrelson said.

To prepare for the role, the free spirit rode along with Los Angeles policemen on patrol, which he said helped him to believe he could play the role.

“And then to jump into the ring with these people (pointing to his co-stars and director Oren Moverman), they make it all much more believable,” he said.

In the film, Mr Harrelson plays a hardened, reckless officer who patrols the streets of Los Angeles, dealing out punishment as he sees fit until suddenly finding himself at the centre of a corruption scandal.

The story is based on a real life Los Angeles Police Department public embarrassment in the 1990s.

Directed by Oren Moverman, who was nominated for an Oscar in 2009 for The Messenger and co-written by crime fiction legend James Ellroy (L.A. Confidential), the film also stars Ben Foster, Robin Wright, Sigourney Weaver, Steve Buscemi and Ice Cube.

When asked about buzz in Toronto that he is deserving of an Oscar nod for his performance in Rampart, Mr Harrelson recalled an encounter with director Milos Forman who had previously pushed him to give a stellar performance in The People Versus Larry Flynt (1996).

“I ran into Milos Forman in New York and he’d just seen (Moverman’s) The Messenger and he said, ‘Now you won’t be able to say that Larry Flynt was your best film’,” Mr Harrelson said.

Certainly, his co-stars think so too.

“Woody is so humble about these things,” piped in Ben Foster. “He’s a national treasure.”

“You see a man who willingly loses his... marbles on the screen and he went there and he continues to go there. He’s one of the finest actors we have and to compare him to anyone else is ridiculous and embarrassing.”