Williams plays the father figure strictly for laughs.

(From Western Daily Press)

Take a film cheerily entitled World's Greatest Dad and boasting Robin Williams in the lead role, and you'd expect something along the lines of Mrs Doubtfire, Happy Feet or indeed any of the family-friendly fare that Williams has lent his name to over the years. But this film is strictly for adults.

"It's a coming-of-age movie," says Williams, 59, sitting in his chair in a surprisingly calm manner, given his reputation for crazy antics.

"But for a middle-aged man," adds the film's writer and director, comedian Bobcat Goldthwait, 48.

Their friendship goes back to the Eighties when they started out on the stand-up circuit and today, they couldn't be closer - literally.

Sitting …