VENICE (Reuters) - John Wayne was the inspiration for Frances McDormand's portrayal of an angel of vengeance in Three Billboards Outside Ebbing, Missouri, the actress said ahead of the movie's premiere at the Venice film festival on Monday.

She stars as Mildred Hayes, who uses three billboards to publicly scold the local chief of police for failing to find the rapist-murderer of her daughter.

"When I was looking for iconic characters in cinema that I might model myself after as Mildred, the only ones I could find were male," she said.

She saw parallels between her character and Wayne's where the protagonists are never just black or white. "I used John Wayne's walk. I can do a good John Wayne," she said.

In the movie, director Martin McDonagh sought to point to all shades of grey, to see "humanity in everyone and not see anyone as particularly the hero or villain".

Mildred, a furious small-town single mother, faces off with the town's revered police chief (played by Woody Harrelson).

He wants to help but does not know how, especially as he is battling with cancer.