Is this the most fun Woody can have without laughing?

The difference between what works and what doesn't this week is simple: vitality. And two old masters, for the moment, have lost it. Woody Allen's Celebrity (even for Allen addicts) seems to be running on empty. More artistic angst, broodings on fame, sexual hang-ups and East Side one-liners. Our familiar writer-hero breaks up his marriage, runs through a string of unlikely women and winds up with nothing (while his neurotic ex-wife mates with a saintly TV producer and discovers Success). But surely, you say, it can't be that bad? The role (the same old role) fits Woody like a glove. He must make something of it?

Alas, no. The original hero is out of sight behind the cameras this time and has, somewhat berserkly, bequeathed his part to Kenneth Branagh who, still more berserkly, attempts a complete impersonation of Woody, rather as though he'd wandered into the Rory Bremner Christmas show by accident. This is a terrible notion, because Branagh is no more a natural comedian than Allen is the Old Vic's dream of Henry V. So Celebrity has a vacuum where its humanity ought to be. The usual constellation of guest stars passes by: Melanie Griffith dispensing blowjobs, Leonardo DiCaprio trashing hotel bedrooms, Winona Ryder loving and leaving. True celebrity, when you think about it, is playing one of Allen's walk-ons for very little money: much cheaper and easier than a stint at the National Theatre. But since, this time, these strolling players don't relate to anyone remotely believable, their little turns are strangely disembodied. Branagh is the centre and it doesn't begin to hold.

There are compensations: there always are. Judy Davis's wife has a mouth of riveting mobility and Woody Allen is incapable of writing a script devoid of decent gags (I counted four). Maybe, next time round, he'll start firing again. The problem, though, could be more deep-seated. Presumably, Allen has decided, after Deconstructing Harry, he can't do lust-crazed 40-year-olds any longer. But only Allen can make sense of playing Allen. He begins to need a whole new act.