Robin Williams: Creep and cheerful

Cinema's Mr Nice Guy has reinvented himself as a twisted baddie and Hollywood's in awe. Calm down, says Roger Clarke. The guy's always been weird. That's why he's so good

Dr Cozy Carlisle. Now there was a character. Not many people remember him from the otherwise indifferent 1991 murder-mystery Dead Again, the film that helped launch the star of Kenneth Branagh and Emma Thompson in Hollywood. Branagh plays the private dick Mike Church, paid to track down Carlisle, a disgraced therapist now packing shelves in a grocery store after being struck-off for sexual assaults on patients. It seems he's been left a small financial legacy that needs delivering. And as Branagh ambles to the back of the store, who should we see lurking there amongst the dark crates of Chianti and the boxed tins of fava beans? Why, Robin Williams of course, wearing manic little round spectacles. In what becomes a performance of shimmering, sinister quality Williams grows easily into the most memorable thing about the movie: a fully paid-up, bona-fide imp of the perverse in an otherwise stagy atmosphere of thespy Branagh-lite weirdness. Branagh too seems drawn to his character, returning superfluously two more times for "advice". Williams's character has little time for niceties. "Fucking do her man, blow her away" he hisses angrily of Emma Thompson, putting a finger gun to his head.

I only mention the marvellously astute Dr Carlisle because there's a lot of loose talk about Robin Williams and his new trio of "dark roles". His murderous turn in Insomnia is set to send shivers down the shires this weekend, while his homicidal kids'-show host in Death to Smoochy will shortly give Krusty the Clown a run for his money. His asocial, voyeuristic loner in One Hour Photo is, I feel bound to say, also a treat not to be missed.

Most people have seen this lurch into darkness as an antidote to earlier bromides like Patch Adams, Jakob the Liar and Bicentennial Man. After all, Charlie Chaplin did exactly this change of direction in his startling Monsieur Verdoux in 1947. This bold move into bitterness (he was a prissy bank clerk given to killing wealthy women) was cordially hated by his key audience. Will the same happen to Williams? Will the modern cinemagoer really take to that nice kind doctor in Awakenings turning nasty? The inspirational teacher of Dead Poets Society going floridly psychotic against the snowy backdrop of Alaska or perving through photographs and stalking a young suburban family?

The thing is, I don't think Robin Williams's career has ever just been "nice". He's always seemed borderline mad to me, openly flirting with the idea of sublimated violence and strange states of mind. His likeable radio DJ in Good Morning Vietnam – chirpy as Mai Lai happens over the horizon – is always skating on the thin ice of loss and pain. Elsewhere, his more broken, sinister side has been obvious, trapped in a haunted board game (Jumanji) or happily acting out a homeless person (The Fisher King) with delusional and occult ideas.

In Hook, he's a harassed banker who forgets he used to be Peter Pan. His return to "boyhood" is immensely disturbing, like a kind of psychotic event. Or how about his freakish old lady drag-act in Mrs Doubtfire? Wearing latex and big breasts to see your own family, by pretending to be the new nanny, doesn't exactly seem something your therapist would advise (and we won't even go near the business of his second marriage, in real life, to a woman who used to be his children's nanny). The tinkling, "magical" music that presages his arrival on the doorstep would be as suitable for the demonic nanny in The Omen for example, which he rather resembles. Freakish child-orientated infantilism is also the theme of the Francis Coppola film Jack in which he plays a boy ageing at an unnatural rate.

He's always had an overblown, endlessly mutating streak of strange behaviour in his act. He's always been the oddball who desperately wants to be liked. Born in Chicago in 1952, the son of a Ford Motor executive who moved his family round as often as any military man, he was by all accounts a solitary boy who seldom had friends for long, preferring instead to live in the world of daydreams in which he made up conversations with imaginary characters. After learning his stand-up shtick at the Comedy Store, having failed to get hired as an actor, he got his first break on TV. He famously auditioned for the part of an "alien from Ork" for the TV sitcom Happy Days by standing on his head when asked to sit down; the character he developed became Mork in Mork and Mindy, which ran on US TV between 1978-1982.

He's an alien who always talked the back legs off a donkey. His talent for talking in tongues, his gregarious glossolalia literally explodes from within him: he can't function unless he's done some obvious improvising somewhere along the line, accessed the babbling brook of personalities that either need a good movie or a good exorcist to contain them. It's the zoo-friendly sounds he makes, and the generous jerks of his arms and legs, rather than the vigorously hairy torso and ugly mug of a face, that bear the burden of seeing him through the business of a full-length movie. In temperament and style he's less like his fellow Hollywood stars and far more like some classic vaudeville turn from the East End of yesteryear, with his straight man's camp, his frivolous love of song and dance and his ability to soak up contemporary references in a net of skilful mimicry.

Like a vaudeville star, like the subject of John Osbourne's The Entertainer, there's are two sides to the grinning stage persona. Obvious signs of a troubled soul were making themselves evident in Williams's life just as Mork and Mindy came to an end. He might appear to be some kind of enlarged hyperactive child, but his tastes were always entirely adult and profane. He developed a serious drugs problem in the 1980s. He was one of the last people to see John Belushi alive after the funnyman freebased all the way to an overdose in the Chateau Marmont hotel in 1982. Williams claimed the shock of Belushi's wretched end was so great he renounced the marching powder for good. It could have been him on the hotel carpet.

He's a performer searching for love. Weird love. Warm love. Anything. He comes out of the closet as a bad guy, then tries to kill us with kindness. The only thing you can be sure of is that no one phase will last for long. He's just switched agents in the past few months, a significant and hard-nosed move, ditching Michael Ovitz who poached him from CAA in 1999 and returning to CAA again, the company Ovitz once headed.

Having ditched the agency that obligingly found him all those "dark" roles, he's bound to be back with the old stuff again. You just know he won't be able to resist that Liberace biopic he's talked about in the past. Expect those ivories being tickled with bejewelled hands, and diamante-encrusted quips for the delight of assorted orphans soon. As for his remake of Kind Hearts and Coronets (with Williams in all eight of the Alec Guinness roles), one can only hope he lets his "dark half" help him reprise that perfect British film, because if he doesn't, the project's doomed. Remember that moment in Hook where he attacks the captain with the words: "dark and sinister man, have at thee!" With Williams, you always know the man he's fighting is himself.