There can't be many plays where actors spend quite so much time with their backs to the audience. For almost fifty percent of the time, Patrick Stewart and Joshua Jackson find themselves doing exactly that – facing upstage in front of Giles Cadle's backstage set, complete with ugly wooden flats, for all the world as if they were wooing some existential audience beyond the back wall. Hang on. Is this a metaphor I see coming towards me - acting as a symbol of the void looming before all of us?

A Life in the Theatre is a bit like that. For all its very hugger-mugger, down-to-earth, greasepaint and sweat practicalities, you can't help feeling pretension is lurking just around the corner. The programme is filled with such authorial dictums as: “I think the purpose of theatre is not to deepen the mysteries of life, but to celebrate the mysteries of life”.

And so say all of us. It also says something for David Mamet's craft that despite flouting “blocking” convention quite so mischievously, this two-hander can still tickle the funny-bone, if less robustly than Michael Frayn's British ode to theatrical existence, Noises Off.

Jackson (“Pacey” of American TV’s Dawson's Creek) making his West End debut is - like Kim Cattrall in the nearby West End production of Whose Life Is It Anyway? - perfectly competent as John, the amiable rookie actor playing second fiddle to Stewart's ageing acTOR, Robert.

A Life in the Theatre, as its name implies, is Mamet's appreciation and ever so slightly affectionate send-up of the thralldom in which theatre holds practitioners and audiences alike. In a series of short, sharp scenes, he takes John and Robert through a dizzying selection of less than brilliant weekly rep warhorses, from World War I over-the-trenches heroics to Old Man of the Sea castaways to cod Chekhov.

Lindsay Posner's direction, however, carries few fireworks, save for its leading man. As Robert, Patrick Stewart, steadily consigning Star Trek and Captain Picard to history, brings a surprising (homo)sexual ambiguity and tenderness to a man seeing the end of his time in the beginning of his younger peer's ascent. Stewart can't do waspishness as Denholm Elliott brought to the role in 1988 or Jack Lemmon did in his famous TV film version. But towards the end, after all the laughter (and there was a lot of it on opening night), you have the odd sensation, despite the physical energy summoned by that wonderful Stewart voice, of seeing him shrivelling before your very eyes.

Like all theatre, it's an illusion, of course. Great acting, though.

- Carole Woddis