‘At this evening’s performance, the title role will be played by …’   Understudy. Somewhere between a trending topic and a dirty word as arguments rage over whether or not audiences should be compensated for missing a favourite star.

Two cases have brought this to a head: at the London Coliseum where Michael Grade’s production of Sunset Boulevard sold £5 million worth of tickets at a good 30% above English National Opera’s usual rates and on the strength of Glenn Close’s name and movie stardom.

At the Menier Chocolate Factory and subsequently on transfer to the Savoy Theatre, Sonia Friedman’s production of Funny Girl has been disrupted by issues surrounding its hugely popular star Sheridan Smith. Speculation on social media (and in the Daily Mail) is rife, suggesting Smith was variously drunk, raving, locked in her dressing room or screaming expletives at her producer. Did the cast actually boo when the house manager announced the show was suspended for ‘technical difficulties’? How ‘technical’ can a show be if you can put it on at the Menier, the simplest of black boxes?

‘Understudy makes good’ is of course the stuff of theatrical legend – ever since Warner Baxter took Ruby Keeler roughly by the shoulders in 42nd Street saying ‘you’re going out there a youngster, but you’ve got to come back a star’ audiences have rooted for the kid from the chorus who made it big. Shirley MacLaine used to sing ‘On Carol Haney’s broken leg I rode to fame’ about her big chance in The Pajama Game and more recently Martine McCutcheon’s repetitive disinclination to turn up in My Fair Lady at Drury Lane made a name for understudy Alexandra Jay, swing Kerry Ellis and replacement Laura Michelle Kelly.

This weekend I was in Verona for a wonderful exhibition about Maria Callas at the Palazzo Forti: even Callas got her break in 1950 at La Scala when she sang Aida as a replacement for the indisposed, and highly indignant, prima donna Renata Tebaldi. She never looked back.

No doubt Callas was a completely adequate replacement for Tebaldi, and Natasha J Barnes is smart and funny and sings well whenever Sheridan Smith isn’t appearing as Fanny Brice – but she isn’t the telly star so many people came specifically to see. Smith is a curiosity though, any criticism of her reliability is instantly countered on social media by a thousand starstruck girls named Kayleigh who appear to speak on her behalf.

The mounting expectations of her young fans must make her work constantly harder. Each time she scores a television success like Mrs Biggs or Cilla (which she told me was the hardest thing she’d ever done), she’s driven by their eagerness and her own determination to ever greater heights. Whilst you can craft these on screen over time with a huge team of writers and directors working around you, the lonely exposure – and the burden – in a stage lead every night must be much more daunting. I can see why anyone would be occasionally stressed by that. Regardless of anything to do with drink, family crises or unsuitable men.

Always ready with the right sarcastic remark for every situation, Noel Coward wrote of actors who persist in appearing despite any setback:

Why kick up your legs when draining the dregs

Of sorrow’s bitter cup

Because you have read some idiot said

‘The Curtain must stay up’

You do feel protective of Smith, though. In fact I want to wrap her in something between a cashmere blanket and a strait-jacket and say ‘take a year off, tend to your dad, sort out your personal life, then come back with a new agent and a new PR team and amaze us all with new creative work.’ At 34, no-one needs to push themselves through pain barriers in the name of musical theatre.

But should you get your money back if the one you came to see is ‘off’? In America there’s no debate – if the name ‘above the title’ doesn’t appear, you can have a refund or rebook for another date. Here, concert promoters don’t think twice about it – if Liza Minelli is indisposed, nobody thinks to substitute her with any of a dozen impersonators who could give almost the same show.  But for theatrical productions, it’s discretionary.

When she took time out from the Menier to be at her father’s side during his cancer diagnosis, Smith so sympathized with fans that she offered to pay for wasted train fares and hotel rooms, guilting her producers into offering ticket transfers to the Savoy run.

Grade/Linnit have so far made no announcement about rebates for performances missed by Glenn Close. When I saw Ria Jones, a disappointed American couple sold me their centre stalls ‘premium’ seats for less than half of what they’d paid, preferring to go out to dinner than to see an actress they hadn’t heard of. As soon as Jones began to croon ‘no more wars to fight, white flags fly tonight’ you could feel the entire audience unclench and relax – she was impeccable in the singing and almost copied Close’s taut and subtly acted interpretation: it would in any other circumstances have been an excellent show.  Just maybe not for £185 a seat.

I see the Americans’ point. Whereas a routine substitution of one actor with another may not materially affect most productions, and many big musicals like Wicked or Phantom are deliberately designed to be resilient to all cast changes – if it’s a big name that comes right at the top of all the posters, I think you should be entitled to something.