Billy Bob Thornton on his Big Bang role

Actor Billy Bob Thornton asked his mother for permission to make a guest appearance on The Big Bang Theory because the sitcom is her favourite show.

Thornton made an unexpected cameo on an episode of the hit show, appearing as an awkward urologist with romantic feelings for Kaley Cuoco-Sweeting's character Penny.

The Oscar-winning actor has told the Late Late Show that his mum Virginia watches the comedy every week and has revealed he asked her for her greenlight before accepting the role.

He also said he was wracked with nerves ahead of making his cameo appearance.

"I was so nervous doing it because I haven't done that kind of show in, like, 20 years," Thornton said.