Billy Bob Thornton had to pretend pal Robert Duvall was his dad for tense courtroom scenes in new movie The Judge - because he couldn't be mean to his friend.

The actor had a hard time prosecuting Duvall's judge character in the film because the two have been pals for years, so he had to imagine he was grilling his father.

Thornton tells Wenn, "Bobby has been my friend and mentor for many, many years. This is the first time I've had to prosecute him, so I think that was probably the hardest part. It's one of the big things for an actor to get over in a movie; if you know someone really well you have to forget who that person is.

"I knew that I was gonna get in his face and that kinda thing, so it was a real challenge to forget who was sitting there. At the end of the day I just put my own father there and did it that way!"

In the film, Robert Downey, Jr's character returns to his hometown to defend his estranged dad (Duvall) when he's suspected of murder.