Understudies, the dependable but largely invisible actors who provide a safety net for West End stars, are being catapulted out of the dressing room and into the spotlight, as growing numbers of stars are unable to keep up with rigorous performance timetables.
Illness, strained vocal chords and additional performances, coupled with high demand for star-studded casts, are contributing to the phenomenon, leaving audiences who have forked out hefty sums for shows increasingly disgruntled.
The problem has led to a flurry of complaints to The Stage, the actors' trade journal, from theatregoers angry and disappointed by the absence of top-billed stars. One, Paul Younger, who attended a production of Wicked earlier this year, only to find understudies going on, said: "When you pay top dollar for a West End theatre seat to see a well-known actor, you have every right to feel in some way cheated if that actor fails to show. How would you feel if you bought a Wembley ticket for a Take That reunion show, only to discover a tribute band on stage?" Some theatregoers have accused West End theatres of a running "two-tier star systems", with understudies going on for matinees. Others have suggested young performers lack the stamina to endure the tough performing schedules.
The actress Gina Beck has leapt to the defence of non-performing stars and their understudies, however. They are an "essential part of the theatre scene", she insists, and it is "naive" to imagine shows would be cancelled when there are "highly skilled understudies waiting in the wings to go on – it's their job".
She argues it is inevitable that performers get sick or take holidays, but says the "show must go on and theatregoers are entitled to value for money, so that's where understudies step in". She dismisses the suggestion that understudies are "second-rate performers", saying that, instead, they bring "energy, enthusiasm and a different approach to the part which often enlivens the whole performance.
"I can understand producers who, seduced by the enormous promotional power of television, seed their shows with famous names. Undoubtedly, this attracts many people to the West End who would not otherwise come, but they come with a different expectation to traditional theatregoers. Should their favourite not appear for some reason, their unfulfilled expectation makes them very unhappy."
She adds: "True appreciation of the role of an understudy requires proper respect for the time-honoured tradition that the show is bigger than any one performer."
Part of the problem is believed to stem from celebrities who have not had the training to handle big performance runs, particularly when singing is required. The former EastEnders actress Martine McCutcheon was among figures panned by the press: after taking the leading role in My Fair Lady, voice strain and throat infections meant she was absent for a large part of the run.
Nicola Burns, a leading producer, called for a "sense of perspective": It is very unusual for last-minute changes and actors are people – not machines. They are just as susceptible to illness as anyone else."
The Society of London Theatre says the decision to cast an understudy normally rests with the show's director and producer. It is at the discretion of the company how and when they inform their audiences. Decisions can be made as late as 20 minutes before the curtain goes up, with a programme flysheet or announcement from the stage flagging up the change.
Last year, the Royal Opera House made history after it opted to refund 20 per cent of ticket prices when the tenor set to star in Handel's Tamerlano was taken ill at the last minute with abdominal pain. That star was, however, Placido Domingo.
One set of people stands to benefit from the friction: the understudies themselves. History shows how dreams can be made in just one fortunate strike. When Laurence Olivier came down with appendicitis in 1966 during a Royal National Theatre production of Dance of Death, an unknown figure stepped into the spotlight and never looked back. He was, as the world would later discover, Anthony Hopkins.
Additional research by Kelly Virdee
Out Richard Griffiths
In Colin Haigh
There was a stunned silence in the auditorium of the Gielgud Theatre in March 2007, when Richard Griffiths, playing the psychologist, failed to appear in Peter Shaffer's play, due to flu. There were further red faces when his understudy appeared not to have learnt his lines, and read the script from a notebook.
Out Placido Domingo
In Kurt Streit
Top German tenor Streit stepped in for performances between 5 and 20 March last year, when Domingo fell ill. Usually, producers defend their stand-ins with the line, "No individual is greater than the company." In a rare step, the Royal Opera House offered a 20 per cent refund on tickets, which cost as much as £210.
Entertaining Mr Sloane
Out Mathew Horne and Imelda Staunton
In Fergus March and Sharon Eckman
When Gavin & Stacey star Mathew Horne collapsed on stage in April 2009, his co-star Imelda Staunton is said to have addressed the audience and asked whether there was a doctor in the house. In the same week, she was forced to pull out of the matinee herself, suffering with a virus. Tickets cost £45 plus.
Sound of Music
Out Connie Fisher
In Sophie Bould
After Fisher was struck down with a vocal injury in May 2007, Bould stepped up from her role as Liesl to play Maria for 12 performances. The 24-year-old was propelled into the limelight, and greeted with rave reviews. Tickets cost between £15 and £40.
The Philadelphia story
Out Kevin Spacey
In Adrian Lukis
In 2005, Spacey was at the centre of a row after theatregoers bought tickets at London's Old Vic only to discover he would not be appearing. He left the role in mid-June for six weeks, to film Superman Returns. The part of C K Dexter Haven was filled by Lukis. Tickets cost between £10 and £40.
Love Never dies
Out Sierra Boggess
In Celia Graham
Graham had an ensemble role in the musical while US soprano Boggess, as Christine, wowed crowds with her Olivier-nominated performance. When Boggess returned home earlier this month, Graham stepped up, impressing both Andrew Lloyd Webber and producer Bill Kenwright. Tickets: £25-£67.50
Out Billie Piper
In Antonia Lewis
Billie Piper missed a preview performance after collapsing shortly before she was due to appear on stage. The theatre manager, who was unaware that she was suffering from cystitis, sent her to hospital. Her absence allowed Antonia Lewis, 25, and previously the assistant stage manager, to make her West End debut.
Out Rowan Atkinson
In Tim Laurenti
Cast in the leading role of Fagin, Atkinson required hernia surgery in March 2009, passing the baton to Laurenti, who was playing the Punch and Judy man. Luckily, Laurenti did not have to go on as Mr Sowerberry at the same time: he was second understudy for that role too. Tickets cost £25 to £72.50.
Out Cassandra Compton
In Zoë Rainey
Producers warned the musical's audiences that leading lady Compton was taking a belated Christmas break earlier this year. However, ticket-holders were furious to find four further principal roles played by understudies. Tickets cost £15 to £65.
My Fair Lady
Out Martine McCutcheon
In Alexandra Jay and Laura Michelle Kelly
When McCutcheon lost her voice in March 2001 in Trevor Nunn's revival of the musical, the leading role was filled so successfully by Jay and, later, in 2003, by Kelly, that it accelerated both their careers. As her understudy, Jay put in more performances than McCutcheon. Tickets cost £25 to £80.