Suddenly, the understudy is the story — less the actual men and women who have to step into the unwelcome spotlight than the phenomenon of a London theatre scene recently plagued by illness, and by unhappy audiences who can be forgiven for wondering just what (or whom) they have paid their £40, £50, even £60 to see.

It would have been possible of late, if you were hugely unlucky, to see Equus, The Sound of Music and Treats without glimpsing Richard Griffiths, Connie Fisher or Billie Piper, all of whom were “indisposed” for varying lengths of time. (Piper has so far been absent the least, in fact: a mere two performances.) Like it or not, there’s no guarantee that a theatre-goer will see the performer who may be the reason they bought tickets in the first place. It’s a bit like buying a train ticket: you can get on, but you aren’t guaranteed a seat. And producers aren’t obliged to give you a refund — it’s at their discretion.

“We’ve always taken the view that, unless it’s a one-person show, it rarely says you’re buying a ticket to see Y artist — it usually says X show,” says the impresario and theatre-owner Howard Panter. What, then, happens if you have Ewan McGregor in Guys and Dolls, as Panter did, then he’s not on — something that, to be fair, happened only twice in nine months? “We try to accommodate at another performance people who feel they did come to see Y artist. In fact, by and large, people stayed and enjoyed it.”

Similarly, Kathleen Turner missed two performances last year of Who’s Afraid of Virginia Woolf? at the start of the run, then a further half-week later on. Her producer, Nica Burns, cancelled the early shows, but takes justifiable pride in the fact that only 10% of the house at any of the subsequent affected performances requested a refund. “When I asked people why they had decided to stay, they said they had heard it was a wonderful production anyway,” Burns says.

It helps, of course, when a show is perceived to be greater than any individual in it. The Sound of Music may have generated its huge advance sales on the back of reality TV, but the ad campaign cleverly plays up the Rodgers and Hammerstein classic, not Fisher. That’s quite different from, say, presenting Madonna in her 2002 West End debut, Up for Grabs. In that case, the Material Girl was the event — and, incidentally, didn’t miss a single show.

Laurence Boswell directed that play, as well as Piper’s current stage debut, so he knows better than most the two-edged sword of star casting. Of Treats, he concedes: “It’s a big part, and the pressure makes anyone more susceptible [to illness]. But I’d wager good money Billie doesn’t miss another show. She’s unbelievably disciplined and hard-working.”

The situation is more stressful for everyone if a star falls ill in previews or just after opening, as with Griffiths in Equus, when a show hasn’t yet found its footing. (Treats had had the benefit of a preLondon tour, so Piper’s understudy was prepared.) Dafydd Rogers, co-producer of Equus, says understudy rehearsals had only just begun on the £700,000 production when Griffiths fell ill within four days of opening, missing seven performances. But Rogers adds that it was “a myth” that Griffiths’s understudy, Colin Haigh, appeared with script in hand; although Haigh “did take some prompts”, the papers he was holding were actually the medical notes belonging to his character, a psychiatrist, Rogers says.

“It’s unfortunate when the illness happens so early in a run,” says the producer Duncan C Weldon, who had Stephen Fry bail out of Simon Gray’s Cell Mates within a week of its 1995 opening. “Obviously, when you’re putting on a play, you aren’t devoting your time to getting the understudy right; you’re devoting it to getting the star right — though that doesn’t negate the fact that you should be fully rehearsed, with an understudy who knows the part.”

The prolific West End producer Sonia Friedman leaves little to chance when it comes to sick stars. “There are many instances where I’ve taken out insurance as well as having an understudy — you can cancel the performance so as not to give the audience anything less than the production they’ve seen reviewed or heard talked about.” She concedes: “People get ill, and there’s nothing, as a producer, you can do about it. There are some actors who’ll crawl on stage when they ought to be hospitalised, and others who find it hard to go on if they have a cold. I know which ones I prefer working with.”

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