One Thursday, Susannah York was Gertrude. Two weeks later, Diana Quick could recite all her lines without a prompt... Jasper Rees offers a short history of last-minute stand-ins

Earlier this month, Susannah York fell off the stage while rehearsing the role of Gertrude for the RSC in Stratford. She broke a bone in her heel and was advised by doctors to take at least six weeks off. Understudies were immediately drafted in to cover her other roles in Camino Real and The Merry Wives of Windsor. But an understudy's brief is to deliver a carbon copy of the missing actor's performance, and as a performance of Gertrude had yet to be created, it could not be copied. So the call went out for a new Gertrude, preferably one with box-office appeal and up to three months on her hands, to start previewing in Hamlet precisely three weeks on from the moment of injury.

Susannah York was injured on Thursday 10 April. Diana Quick heard from her agent that she'd been offered the role the following Tuesday. By Wednesday, she had extricated herself from a Radio 4 production of War and Peace, sorted out child-care (her partner, Bill Nighy, is also away from home, touring in David Hare's Skylight), and accepted. But she could not arrive in Stratford until the Thursday evening, and began rehearsing only on the Friday, eight days after the vacancy fell open and 14 days before the first preview. By last Sunday's run-through, she was able to recite the whole part without a prompt, despite the fact that the director (Matthew Warchus) had enlarged the role by rescuing a scene with Horatio from the First Quarto and stealing a few lines from Claudius.

Warchus himself was less fazed by the shortage of time to rehearse his new Gertrude. Two years ago he lost Una Stubbs from the cast of Peter Pan during the technical rehearsal. "That was even more harrowing," he says, "and it's given me some reserve for dealing with the situation.'' And Quick was moderately well prepared: she has twice played Ophelia, has turned down Gertrude three times, and has previously stepped into the breach at far shorter notice than a fortnight.

"In my youth, I once took over in a play called Mind Kill at Greenwich by Don Webb," she says, "which I had been offered initially and had declined to do, so Jill Bennett was doing it instead. Then there was a disagreement and Jill left the cast and they came back to me. I did that in a week. It was the most terrifying thing I have ever done - it was a new play, a gigantic part - just in terms of the feat of memory, let alone trying to put a performance together. Things have to cook: that's the dangerous thing about doing something in a short period."

Actors step belatedly into productions almost always as a result of another actor's indisposition. And, like Quick, they have to learn at pace. When Simon Russell Beale overdid the crookback as Richard III and slipped a disc, he was replaced for the RSC transfer to the Donmar Warehouse by Ciaran Hinds, who rehearsed the role in 10 days. After Stephen Fry absconded from Cell Mates, Simon Ward was performing the role of George Blake on a week's rehearsal.

In extreme cases, it's not indisposition that forces cast changes but death. Leonard Rossiter died during a run of Loot, and was replaced in haste by Dinsdale Landen. Ralph Richardson's death left a gap in the cast- list for Eduardo di Filippo's Inner Voices, at the National Theatre, which Robert Stephens, already a member of the company, took over.

This nether world of accidental substitution appears to have no laws. Someone falls ill; someone else steps into their shoes. But you occasionally chance upon curious patterns in the apparently random malevolence of fate. Laurence Olivier once had to withdraw due to ill health from Love for Love at the Old Vic, and Derek Jacobi, a notably quick learner, stepped in. His late arrival in the role then was mirrored last summer by an early departure from the same play at Chichester, when Jacobi succumbed to appendicitis after one preview. Owing to pregnancy, Samantha Bond withdrew from a recent RSC production of As You Like It: Kate Buffery, Celia to her Rosalind, replaced her, and after giving birth Bond returned to the production as Celia. The director Michael Rudman was once removed from a production of The Admirable Crichton at the Haymarket, reportedly at the behest of the star Rex Harrison. Duncan Weldon, who produced the show, recently made reparation by inviting Rudman back to Chichester to direct the play with Ian McShane. It opened last week.

Indisposition is often merely the cosmetic reason for an actor's withdrawal. The sore throat mentioned in the press release can cover for a nervous breakdown, pneumonia for a chilly relationship within the cast, exhaustion for alcoholism. Very rarely is there a frank admission of frailty, because that might endanger an actor's future. It's usually the interlopers, with no theatrical career at stake, who are honest about their weaknesses. Daniel Day-Lewis withdrew from Hamlet publicly citing a nervous breakdown, and was replaced by Ian Charleson (who less publicly had Aids and himself had to withdraw occasionally from the show). Roger Moore chose to bow out of Aspects of Love during rehearsal, making way for Kevin Colson, when he discovered he couldn't sing. And most famously of all, when Fry escaped from Cell Mates it was in his interest to advise the show's insurers that he had had a nervous breakdown, freeing him from liability for losses of pounds 500,000.

Some shows appear to thrive on the creative tension caused by serial firings. In 1990, Richard Harris toured the provinces in Pirandello's Henry IV in which the rehearsal period saw the departure of no fewer than three directors: the original incumbent Philip Prowse was replaced by Chistopher Fettes, who made way for David Thacker, whose place was taken by Val May. The designer Sheelagh Keegan left, to be replaced by Tim Goodchild. And the female lead Sarah Miles was replaced by Isla Blair (a serial substitute, by the way: she also took over from an ill Nicola Pagett in the National's What the Butler Saw). These ructions were fully reported in the press and, as the show prepared to enter the West End, a calamity was expected. The critics raved about it and Harris won the Evening Standard award for Best Actor n