Accept no substitutes?

I'd been to the box office, I'd phoned and dogged the theatre's website like a shadow. When a couple of tickets to the RSC's sold-out Hamlet came up, I pounced, and rejoiced in my luck. The next day came news of lead actor David Tennant's injury and I took it badly. Like everyone else who will see the first few weeks of the production's London run, I caught Edward Bennett, bumped up from Laertes to take the title role.

In some ways, we were fortunate: Bennett was far more than merely well prepared, he gave an eloquent, angry reading of the role, apparently quite distinct from Tennant's. He's a stolid baritone of an actor (as the king of Navarre in Love's Labour's Lost, he made a good dogged foil to Tennant's febrile Berowne), and his was a thoughtful, determined Hamlet, even if short on wit, edge and lurching instability.

Should it matter? As someone who habitually only gets round to booking shows as they're about to close, I've often found acclaimed performers replaced by understudies or wholly recast. Catch even the great Michael Gambon some time after opening and you may as well get an understudy: I still remember watching his Volpone and regretting that his absorption in the role seemed to have melted into a get-me-out-of-here gabble. No one is indispensible, of course: ballet-goers in particular are used to finding a slip in their cast sheet announcing that, due to injuries almost everyone on stage seems to be standing in for someone else.

Is that what theatre is: understudying for life? People pretend to words, emotions, actions that aren't their own, while we sit in the dark putting our own lives on hold to invest all our concern in the fiction. It's pleasurable - often, let's face it, a relief - but also necessary. Theatre as a substitute for the existence outside may not merely give us a chance to escape our lives, but also to think about them more clearly, more profoundly.

That may not be much consolationwhen you want to see Dr Who. There are times when the show is the star, and a substitution may give a role a new gloss, it doesn't alter a strong directorial vision. At times, understudying is almost the point: Chris Goode's fascinating experiment ...Sisters at the Gate Theatre last year was a radical, partly improvised, re-distillation of Chekhov's last play. Each performance began by the six performers being allotted their roles, which might differ every night. This radical substitution was perfect for a text in which virtually everyone feels they've been denied the life they deserve. The sisters and their friends all feel that they've been miscast, subject to random acts of disappointment, if cut by brief shards of promise.

The RSC Hamlet is certainly not that kind of show. So what difference did Tennant's absence make? Although some commentators lament that audiences are dangerously in thrall to star power, we can't pretend that the casting of the central role isn't crucial in a play like Hamlet, in which so much is prismed through the prince's consiousness. It's particularly true of this production. Doran is a sensitive director, and often an interesting one, but his readings of Shakespeare are often built around a strong leading performance - by Antony Sher in a series of productions, by Harriet Walter and Patrick Stewart in Antony and Cleopatra, and by all accounts by Tennant in Hamlet.

Doran's Elsinore delves into how relationships curdle within the family and friendship, but only fitfully relates that to a broader vision of the world. The prominent figures here have all made uneasy compromises with pragmatism: Patrick Stewart's usurper is a man who has longed for power for years, only to find that, once attained, he doesn't really know what to do with it. Penny Downie's queen is accustomed to compensating for her husband's gruffness - remembering people's names, making them feel important - while setting strict limits on what she allows herself to know. Oliver Ford Davies is an excellent Polonius, a spymaster sliding into senility - but although the production excellently shows him turning surveillance on his own children, it seems less interested in his governmental function. He is eventually shot when skulking behind a mirror, which cracks like a spider's web: a nice visual metaphor for an Elsinore we haven't really seen.

To galvanise all of these careful performances, we need a risky, questing Hamlet, a man who will always go too far, grieve too much, ask too much. The first act ends with an unexpected shock (don't worry, no spoiler here): but it depends on our believing that the prince might do something bloodily impulsive. The moment sat awkwardly on the substitute prince - if Tennant's Hamlet is a maverick existential detective, Bennett is a diligent copper, and that's not enough to galvanise this production. Some things you can't replicate.