Interview - Robin Williams: Funny guy is seriously back on top form

Not much fanfare accompanied World's Greatest Dad when it premiered at the Edinburgh International Film Festival this summer.

In fact, it would be fair to say that Robin Williams, for so long the anarchic comedian of choice for anyone remotely interested in painful belly laughs, had slipped into the category of box office poison.

Williams had joined Steve Martin and Eddie Murphy among the former box office titans who had gone off the boil. Then along comes World's Greatest Dad to remind us all how seriously funny he can be while simultaneously tugging at audience heartstrings.

The movie has everything against it. Completed some time ago and sneaked out in the US in 2009, it emerges onto the UK circuit as just another low-budget indie with, one assumes, a washed-up star at its core.

The assumption is very wrong. Williams is excellent as Lance Clayton, a man drowning under the realisation that his life is dysfunctional, frustrating and impossibly dull.

And just as he proved in Dead Poets Society, Good Will Hunting, One Hour Photo and Insomnia, he can be mesmerising when handed the right material.

World's Greatest Dad reunites Williams with Bobcat Goldthwait, the comic talent best known for his turn as the vocally challenged, wild-haired, wild-eyed Zed from the Police Academy series. Since those far-off days, Goldthwait has emerged as a director of promise with three acclaimed films.

Williams (billed as Marty Fromage) agreed to do a small role in Goldthwait's Shakes the Clown in 1991. He initially looked at the script of World's Greatest Dad with a view to contributing a cameo. In the event, he took the lead role.

"I've never felt as safe as when working with him," says Williams of his 30-year friendship with Goldthwait.

"I know him as this really kind, ethical guy who's really funny but at the same time so loyal and so sweet. Everyone thinks of him like they think of me: 'Ooh, he's crazy!' (But] that's what he did a long time ago. I read it and I went, 'I know you wrote this, and I know you're a good filmmaker, so okay, I know where you're going with this.' I knew he wouldn't hang me out to dry. So it was cool."

World's Greatest Dad turns on a tragedy that neither Lance nor his sex-obsessed, potty-mouthed son, Kyle (Daryl Sabara) can predict. Lance is a failed writer who finds himself trapped teaching poetry to bored high school students.

But his ambitions are resurrected when he is caught within a terrible heartbreak that allows him to offer his child a father's ultimate, unconditional love while simultaneously exploiting the situation for his own ends.

Now 59 and the father of three children, Williams grasped Lance to his heart.

"I just resonated with him as a parent – knowing those moments where you try to connect. So it's that thing of trying to connect with your son, trying to find common ground, and at some point finding common ground and at other points missing.

"And the fact this boy played him so unrelentingly nasty made this movie work, because if he's not a p***k, then when you eulogise him it doesn't work."

Williams's work in recent years has not been particularly memorable. Like Steve Martin, who segued into edgy drama with Novocaine and Shopgirl, Williams has found his way back to quality movies via the independent road. He admits that some observers were surprised by the change.

"It was actually a saving grace for me," he smiles. "It's the type of thing I need to do more of. There are some movies you do, obviously, for the money; but doing movies like this is what really gives you satisfaction. It's freeing in wonderful ways.

"I read it for Bob as a favour, and it was so good. And it's weird because my being in the movie didn't give him a huge budget, but it did give him a little more scratch to make the movie."

Williams' career has flatlined in recent years. Not on the scale of Mel Gibson's meltdown but, nonetheless, Williams has lost much of

the box office clout he once had.

Yet despite the drugs, the drink and the vagaries of the showbiz world, he is still punching his weight. It never fails to surprise him. "Yeah, fame is a weird animal. And the isolation is real. That's why some people end up living, like Michael (Jackson], alone in a mansion, doped out of their t**s," he says with a mixture of aggression and raw candour. "That's how you sleep: you're using a drug that people use for heart surgery.

"The isolation, that is pretty frightening. And the more you get, you find yourself living behind a gate in a strange place. Or you get everything you want and then you realise, 'Is that everything you want? Is that everything?'

"Money can't buy you happiness but it can buy you s**t and a lot of good times. Yes, understood. But it's the other thing of, what really works on that level? And if it's built around a lie, then what the f***...?"

World's Greatest Dad (15) is on nationwide release.