John Kolvenbach's On an Average Day is only an average play, but it is at least partially redeemed by the live wire, high-wire acting duet it affords to two terrific American movie and television actors making their British stage debuts.

After such recent celebrity outings on the London stage as Madonna (who couldn't act), Matt Damon (who looked pretty and acted eagerly), and Gwyneth Paltrow (who was suitably morose, but marooned in an inferior play), here are two more: Woody Harrelson and Kyle MacLachlan, both of them quirky comic actors of apparently limitless charm.

There is something about the gangling, loose-limbed Harrelson that instantly commands attention, and it's not just his boyishly handsome features offset by the grown-up receding hairline. Best known for playing the barman Woody in the long-running television series Cheers, the real-life Woody has lately been making a name for himself by getting physical in London cabs, and in his amusing programme biography, he makes reference to it, writing that loves being here, "playing football in the park, taking late night taxi rides and rubbing elbows with policemen."

Harrelson's body language is perpetually that of the playful puppy who might turn vicious; and so it proves here, as his character Robert spars with his older brother Jack, who has returned home after a 23-year-absence.

As the brother, MacLachlan (currently best known as Charlotte's estranged husband Trey in the fabulous Sex and the City, but also remembered from his collaborations with David Lynch including the TV series Twin Peaks and movie Blue Velvet) is the perfect complement to Harrelson's lack of containment, offering a study in buttoned-up, uptight emotion.

There's a legacy here of abandonment - both brothers were left by their father when they were 15 and seven respectively, a story repeated by Jack who himself left young Robert. But today Jack has returned, and the stage is set for the recrimination and rage of revisiting the past.

Kolvenbach's play is a not very subtle rewrite of Sam Shepard's sibling tale-of-opposites True West, complete with a similar incident of cutlery being strewn on the kitchen floor. As designed by Scott Pask, the kitchen is a chaotic mess of fading cuttings and piles of old newspapers, and the fridge holds a particularly nasty smell that gives the play an overstretched running joke.

Were it not for the presence of these actors, On an Average Day would be a fringe event at best; but Harrelson and MacLachlan give it a much more compelling sheen under John Crowley's direction.

- Mark Shenton