First Night: The Little Dog Laughed, Garrick Theatre, London

To describe the handsome Rupert Friend as Keira Knightley's current squeeze may sound a touch disrespectful, rather as if one were to refer to Simone de Beauvoir, say, as Jean-Paul Sartre's arm-candy. But that's what he is, lucky guy, and here he is making his high-profile professional stage debut as one of the leads in Douglas Carter Beane's bouncy Broadway comedy The Little Dog Laughed. At least Keira will know where he is in the evenings while, a few West End streets away, she is breaking her own theatrical duck in style in The Misanthrope.

Nor does she need to worry that female love interest onstage will turn into an offstage romance, for her beau is playing Mitchell, a dashing boy-next-door type actor and aspiring movie star, whose professional secret is that he is gay. And in the course of the proceedings, our hero falls in love with a male prostitute who hitherto had always thought of himself as a straight guy servicing other guys merely in order to pay the rent. Indeed, the reluctant male hustler (played with a witty, winning sensitivity by Harry Lloyd) occasionally sleeps with a lovelorn – and underwritten – female friend, who is portrayed here as a nice mix of the ditzy and the distressed by Bond girl Gemma Arterton.

The character who powers the evening forward, though, in Jamie Lloyd's enjoyably breezy and knowing production, is Diane, Mitchell's diabolically cynical lesbian agent. She makes the average monstre sacré look like a fluffy kitten in a cat-food advert. If there were an Olympic decathlon for bravura bitching and Machiavellian scheming, she'd win the gold, silver and bronze. And in a performance of brilliantly malign energy, Tamsin Greig shows you a woman who has a well-thumbed movie-industry directory where her heart should be.

"Are you British? Do you have a knighthood? Then shut up!" she bawls at her client Mitchell, whom she wants to keep firmly in the closet. To purchase the rights of a Broadway play for him, she travels from LA to New York, though "theatre" is the kind of word that makes her mime throwing up (she's big on showing disgust through camp italicised stresses and over-the-top incredulous intonations). But ironically the character could also be construed as the bravura, flouncing Spirit of Theatre, as she mockingly performs the role of MC in Little Dog Laughed and, like some satanic surrogate- playwright, steers the action towards the phoney happy ending of a PR-dream "heterosexual" wedding.

Before last night's opening, I sat on the Tube opposite a man reading The Sun. On the front page, there was the headline "Walliams Weds" and a message from Kylie that she fancies girls. Walliams is the doyen of the kind of teasing metrosexuality that never gets a proper look-in this play where the characters seem to be locked into slightly dated notions of what is possible.

At one point, Diane talks about movie executives who out-refine themselves in their fussiness over their lunchtime salads; equivalently, there's an increasing danger that a comedy as relentlessly in-the-know about showbiz amorality as this will eventually outsmart itself. The scenes that attempt emotional seriousness ring hollow and false. The thin characterisation of the young girl suggests that the author is much less interested in straight, sincere females than he is in dyke divas.

And there seems far less at stake here than in, say, Take Me Out, the American play at the Donmar a few years back about the exposure of a gay baseball superstar. But the production has a lot of diagrammatic pep and Tamsin Greig is phenomenal as corrupt Hollywood ambition incarnate.