Film: The Big Picture - Mirror, mirror, on the wall, who's the vainest of them all?

CELEBRITY (18) DIRECTOR: WOODY ALLEN STARRING: KENNETH BRANAGH, JUDY DAVIS, WINONA RYDER, LEONARDO DICAPRIO 114 MINUTES

Celebrity is a mask that eats into the face," wrote John Updike. "As soon as one is aware of being `somebody', to be watched and listened to with extra interest, input ceases, and the performer goes blind and deaf in his overanimation." Woody Allen's sprightly new picture Celebrity goes some way to bearing out these observations. It's about a self-absorbed celebrity journalist named Lee Simon who dumps his wife after 16 years of marriage in order to pursue younger and more glamorous women.

Five minutes into the story and an actress (Melanie Griffith) is down on her knees plucking at the buckle of Lee's tickled trousers. Next he's getting all steamed up with a blonde supermodel (Charlize Theron), who breathily informs him that she's polymorphously perverse and has a mirror on her bedroom ceiling.

Time was when Allen himself would have taken Lee's role - the schlemiel who can't believe his luck; this is the same man who once confessed that the reason he got into movies was to meet girls. But after the world went "yeeeugh" on seeing him kiss Julia Roberts in Everyone Says I Love You, he finally twigged that he was too old to play the Lothario. In Celebrity he has wisely chosen to stay behind the camera.

Rather less wisely, he has recruited Kenneth Branagh to do a Woody Allen impersonation. In a recent Parkinson interview Allen spoke warmly of his star's performance, but I don't think many others will share his enthusiasm. Sure, Branagh has got every last stutter, gulp and hand gesticulation down to a T, but you are never allowed to forget the performance and believe in the character.

This miscasting might have been calamitous for the film were there not a parallel story concerning Lee's ex-wife Robin (Judy Davis), a pinched and desperate woman who can't get over being abandoned; in her flailing attempts to calm herself she goes on a convent retreat, considers plastic surgery and takes sexual instruction from a prostitute (Bebe Neuwirth, in a superbly matter-of-fact performance). "What goes through your mind when you do it?" "The crucifixion," Robin replies, unsmiling. Davis, whose prickly distress was the best thing in Deconstructing Harry, goes so deeply beneath the skin of this woman that it's almost painful; I don't think it would be controversial to call her the most intelligent screen actor of her generation.

A host of celebrity cameos satellite around Lee and Robin, lending the film a bustling, near-Altmanesque crowdedness. Joe Mantegna plays nicely against type as a suave TV producer, Winona Ryder is an aspiring actress, and - there's something for everyone here - Leonardo DiCaprio sends himself up as a spoilt movie brat who slaps his girlfriend around, trashes his hotel room and offers Lee exclusive access to his orgy. (I wonder: does this answer to the experience of any celebrity interviewer in the world?).

Lee, his appetite for nubile flesh still unsated, eventually joins in, though his real reason for hanging on to the brat's coat-tails soon becomes clear: he wants to sell his screenplay. Indeed, as the film proceeds, it seems that it's less about celebrity than ambition. Lee doesn't hang out with the beautiful people just to feel good about himself - he's a professional climber, too. After two poorly reviewed novels, with a third unfinished, he's decided to tip his hat at Hollywood, where the bucks flow and nobody minds if you write crap.

But the problem of Branagh won't go away. While Lee is meant to be covetous and unamiable, Allen means us to engage with his fate - much as we did with a previous surrogate, John Cusack, in Bullets Over Broadway.

If Allen had gone out on a limb and cast someone like Steve Buscemi in the role, he might have forced us into a perverse sympathy - for an earthling among the Venusians. Branagh, unfortunately, is neither one thing nor the other, and you certainly have no idea why young women keep throwing themselves at him. His break-up scene with a book editor (Famke Janssen) who believes in his talent should be poignant as well as funny - removal men lurk in the background as he explains why he can't live with her - but there's nothing for the audience to respond to in Lee's self-loathing. (We got to the loathing bit long before he did.)

Shot in black and white by the Bergman veteran Sven Nykvist, the film has the lovely, woozy look of a half-remembered dream; Janssen throwing pages of a manuscript from a drifting ferry is perhaps the most poetic sight of all. There's a kind of elegance, too, in its symmetry: while Lee goes into freefall, Robin becomes a reluctant success as a daytime TV interviewer. Celebrity has been more or less thrust upon her: "I'm everything I ever hated," she says, "and I've never been happier." It's not clear if we're meant to fear for her future. Will she become, in Updike's phrase, "blind and deaf in overanimation"?

Allen mocks the fame game here, often wittily, yet since he's a famous person himself it's tempting to wonder how reliable his reflexes are. Is it really the case that everyone becomes tainted by celebrity's touch? Can't it be survived? Personally I prefer Steve Martin's less pessimistic definition of the state: a celebrity is simply a movie star who looks as if he spends more than two hours working on his hair.