Robin Williams talks about his latest movie, World's Greatest Dad, with friend, fellow comedian and the film's writer and director, Bobcat Goldthwait, in the run up to its release on Friday, September 24. The actor talks about the role not being typical Williams fare, what it was like working with a brilliantly obnoxious 'son' and why he felt the need to get naked in the film.

By Susan Griffin.

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 1980s when they started out on the stand-up circuit and today, they couldn't be closer - literally.

Sitting side by side, Goldthwait has his arm draped around Williams's shoulders for the duration of the interview, and while they couldn't be more differently dressed (Goldthwait in jeans, checked shirt and a white flat cap, Williams in a grey suit and black shirt) their camaraderie says it all.

If they're not finishing one another's sentences, they're chuckling at what the other has just said, no more so when there's a crude joke or two involved.

It gives you an inkling as to what to expect from World's Greatest Dad, a film hailed as 'lusciously perverse' when it was shown at the Sundance Film Festival in 2009.

But what else would you expect from Goldthwait, a man who's brought us Shakes the Clown and Sleeping Dogs Lie, which Williams describes respectively as "the Citizen Kane of alcoholic clown movies and a film about a girl who gets 'close' to her dog," much to his mate's amusement.

There's more laughter when asked if people may be surprised by Williams's involvement in the black comedy.

"What? Like how did he get you to do that'?" asks Goldthwait.

"'Did he have pictures of you and the goat'?" says Williams with a short whistle and a crick of the neck, and the pair continue to drum up fictitious scenarios as to how Williams could have been coerced into shooting the film.

"I'm not surprised, but maybe others are," says Williams, who also starred in Goldthwait's Shakes the Clown back in 1991.

"Actually Robin read the script for World's Greatest Dad and was going to help me out and play a small part," says Goldthwait.

"Then he called and said, 'Can I play the lead?' I was like, 'Woah!' I mean Robin's one of the finest actors in the world, so to have him want to be [the lead] in my movie changed everything."

Williams can't help but make a face as if to say 'shush now'.

"I didn't do this as a mercy thing, like, 'Oh, I'll help little Bob'," says Williams.

"I really thought it was well written and I also know him as this really savvy film guy."

In the movie, Williams's character is Lance Clayton, a single father who's learned to settle, having once dreamed of being a rich and famous writer, but only managing to make it as far as a high school poetry teacher.

He's dating Claire, the art teacher, who is adorable but won't acknowledge publicly that they're dating. And then there's his only son, Karl.

Credit must go to young actor Daryl Sabara, who makes Karl so insufferably obnoxious, it's almost painful to watch.

It's a world away from the Spy Kids trilogy Sabara starred in, but then Goldthwait didn't know about those films when he cast him.

"I'd never seen them. It would just be creepy for a middle-aged dude to watch Spy Kids movies," he says.

"He was actually coming in to play Andrew, but lied and said he was there to audition for Karl."

"He's amazing, he's fearless," adds Williams. "In real life Daryl is a sweet kid but plays Karl as a little p***k and unrelentingly so!"

Williams is known for improvising on set and it was no different in this film, though he adds he wasn't just "going for jokes".

"Darryl and I just played back and forth, like rough play. He would say nasty things [and I'd react]," says Williams.

"Robin's very kind about the script, saying it's well written, but it really was collaboration. That's how I like to work," adds Goldthwait.

"If people come up with an idea, I'll let them try it."

In the wake of a freak accident, Lance suffers the worst tragedy and the greatest opportunity of his life, bringing into question the subject of honesty.

It's a theme Goldthwait also explored in 2006's Sleeping Dogs Lie.

"World's Greatest Dad is the flip side to that movie," says Goldthwait.

"The other film looked at how honesty may not necessarily be a good thing in a relationship, I mean total honesty, total disclosure," explains Williams.

"Whereas this film is saying, 'the truth shall set you free'."

And it does, for Lance, who celebrates this realisation by shedding all his clothes and diving off the top board into a school swimming pool.

A liberating moment for anyone, as Robin Williams is now willing to confirm.

Extra time - World's Greatest Dad :: It was shot in Seattle: "A really wonderful, strange place to shoot and a perfect fit for this movie," says Williams.

:: Goldthwait based the character of Lance on himself.

:: Krist Novoselic from the band Nirvana appears as the newspaper vendor who consoles Lance.

:: Given the Nirvana link, many believed the scene of Lance floating in a swimming pool was a homage to the band's album Nevermind.

:: At the Sundance Film Festival, Williams's performance in World's Greatest Dad was hailed as "outstanding".

:: Editors: Please note language in par 25.

:: World's Greatest Dad is released in selected cinemas on Friday, September 24.