Title Sweeney Todd Actors Greer Grimsley, Luretta Bybee, George Masswohl Company Vancouver Opera Venue Queen Elizabeth Theatre City Vancouver

From the stage of the Queen Elizabeth Theatre on opening night of Vancouver Opera's Sweeney Todd, conductor Jonathan Darlington knew there were problems "from the very first note" sung by the show's star, Greer Grimsley. By the time Grimsley, as the mad barber, had sung his duet with Mrs. Lovett (played by his real-life wife Luretta Bybee), it was clear that the bass-baritone was in trouble.

"While he was literally going voiceless on the stage, I was looking into the wings to see if they were telling me to bring the curtain down, because of course I had no communication with anybody," says Darlington, who had the orchestra with him on stage rather than in the pit.

After the duet, Grimsley walked past Darlington to get to his next position on the stage. "I whispered to him, 'So what do you want to do?'" says Darlington, recalling the night in late April. "And he said, 'I don't know.'"

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In theatre lore, this is the moment when the understudy steps in and a star is born. But in reality, most performing arts companies in Canada do not, as a rule, employ understudies, covers or swings (who learn several roles) consistently. While there are exceptions – the Stratford Festival and Toronto's Mirvish Productions among them – smaller companies do not have sufficient resources.

"Hardly any company of our size does," says James Wright, general director of Vancouver Opera. VO usually uses members of its young-artists program to understudy leads, but there was nobody in the program trained to sing Grimsley's role, and no understudy had been hired before rehearsals. "It's almost like double-casting," says Wright. "You pay them a fee to sit here."

"The organization around it is tricky, and the rules around it bring an element of unwieldiness," says Christopher Gaze, artistic director of the Bard on the Beach Shakespeare Festival in Vancouver, which also doesn't use understudies. "There's a lot of hope for the bloody best."

In the week before opening Sweeney Todd in Vancouver, Grimsley alerted the company that he wasn't feeling 100 per cent. Then, around 4 p.m. on the day before opening night, singer George Masswohl – who was sitting on his couch in Stratford, Ont., doing his taxes – got a call asking if he could fly to Vancouver as a possible sub for Grimsley. Masswohl hadn't sung the role in 12 years, but the next morning he was on a flight, reviewing the score. When he landed, Grimsley said he was feeling okay, so when the curtain rose that night, Masswohl was settled into his seat to watch the show.

Not for long. Once Grimsley began to sing, Masswohl heard the problem: "I immediately sat up and went 'Oh goodness, this is actually happening.'" He took off for the lobby, an usher warning him that he could not return if he left. "I said, 'I won't be coming back in.'"

Masswohl was rushed backstage, where he was set up in the wings stage right with a microphone, a music stand and a light. Darlington figured out what was going on after a few notes (at first both Grimsley and Masswohl were singing, until Grimsley realized he could stop). After that, Grimsley continued to walk the part, but only lip-synced the songs.

An announcement about the change was made after intermission.

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It was hardly seamless, and Masswohl's musical-theatre baritone was noticeably different from Grimsley's opera bass-baritone (Grimsley delivered the spoken dialogue). But under the circumstances, it was a remarkable recovery, and Masswohl's opera debut was unforgettable.

"I've experienced a lot in 30 years in show business, but this takes the cake," he says.

Calgary Opera also experienced a cake-taking in April with its production of Carmen. Mezzo-soprano Sandra Piques Eddy, who reported a tickle in her throat after dress rehearsal, sang the first aria on opening night, walked off stage and said she didn't have anything left, says Calgary Opera general director and CEO Bob McPhee. "If they keep pushing like that," he says, "we're talking damage."

Beste Kalender, a member of Calgary Opera's emerging-artist program who was cast as Mercedes, covered the role, singing from backstage while Eddy walked the part. By the second performance, when Eddy had come down with a full-fledged flu, Kalender stepped fully into the role.

It got worse. After that performance, Antoine Bélanger, who was singing Don José, said he wasn't feeling well, and soon he was "sounding more like a bass than a tenor," says McPhee. "They're spitting on each other and they're kissing each other, so these things are bound to happen." McPhee flew in another tenor from out of town.

So Eddy and Bélanger were onstage walking the parts while two covers sang offstage. Meanwhile, conductor Tim Vernon also came down with the flu and had to conduct sitting down.

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Despite the Carmen calamity, McPhee is not considering employing understudies for future productions. "It's astronomically expensive," he says. "And in our case, when you look at the cost versus the risk – once in 17 years – we don't have those resources."

Unlike budget-strapped regional companies, the Canadian Opera Company generally does use understudies. "In order for myself and for Alexander [Neef, COC general director] to sleep at night, we always have a plan," says artistic administrator Roberto Mauro.

The plan, however, can vary from opera to opera. The understudy might be a member of the chorus, or, if in rep, a singer from the other production. It could also be a member of the other cast if the show is double-cast, or the COC might bring in a singer from elsewhere.

The costs to employ understudies vary widely: Canadian Actors' Equity Association has 21 agreements and policies in place across the country, with different costs depending on factors such as genre and company size. At Mirvish, for example, a full dress rehearsal must be held for the understudies, including the rest of the cast – generally after opening.

"It's an expensive procedure, but it's necessary in order to guarantee the audience will get the show they paid to get, unlike what happened in Vancouver where they saw one person on stage and another person in the wings singing the role," says John Karastamatis, Mirvish's director of communications.

In commercial theatre, understudies and swings are a fact of life. Mirvish has a full complement of understudies ready to go on at a moment's notice if something goes wrong – as it did this year during a performance of The Heart of Robin Hood. When the actor playing Marion hit her head during the show, the stage manager immediately dressed the understudy, who went on for the next scene. The show continued without a pause.

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"Understudies love to go on because, otherwise, they feel like they have no purpose," says Karastamatis. "It's not like the kind of job where you're hoping never to go on. Nobody becomes a performer just to perform alone, or, as the saying goes in the theatre, it's like practising kissing with your sister."

No one is saying understudies wish the actors they're covering any harm, but a well-timed appearance has been known to do wonders for a career. At the Stratford Festival in 1956, understudy William Shatner famously went on for Christopher Plummer in Henry V and subsequently rocketed to fame. "Bill was incredible," Plummer recounted years later. "He not only knew the part backwards, he did things that I hadn't done. He stood up when I sat down. He sat down when I stood up. He was extraordinary, that son of a bitch."

These days at Stratford, an understudy task force is looking into the system "just to make sure what we're doing makes sense," says artistic director Antoni Cimolino, himself an understudy for Colm Feore in Molière's The School for Wives in 1991, when Feore was hurt in the annual Stratford-versus-Shaw Festival cricket match.

"I saw him go down and it was serious – he was not getting up, and the ambulance was called," says Cimolino. "Everyone else was crowding around Colm; I was going away to look at the videotape of the Molière play, because I knew that within 24 hours I'd be on stage."

Cimolini figures his performance had something to do with the fact he was selected to play Romeo the following year. "It's a great chance for a young actor to take on a lead part," he says, adding: "Suddenly you're looked at in a different light, because you're able to hold the stage."

Still, every year at that cricket game, he says, "part of me just flinches."

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Some companies deal with understudies on a case-by-case basis. The Arts Club Theatre Company in Vancouver does not usually employ understudies, but had a chorus member understudy the lead for Mary Poppins this season and last – due to the longer run of the show and the prominence of the part.

"If you've lost Mary, you've kind of lost the show," says artistic managing director Bill Millerd.

In 2011, when Jay Brazeau had a stroke during a preview performance of the Arts Club's Hairspray, a replacement was quickly found. Andy Toth learned the part in two-and-a-half days, performing the first show holding the script. The company lost only three shows.

"It's been very rare that we've actually lost performances," says Millerd. "With shorter runs, you're hoping that you'll get through it somehow."

At Bard on the Beach, which does not use understudies, Gaze talks about "Dr. Theatre" – how getting on stage to perform a part can help an actor deal with what ails them. But even Dr. Theatre fails to come through at times, and Gaze himself has stepped into roles, playing everything from Petruchio in The Taming of the Shrew to the queen in Cymbeline – going on in his beard and without a dress (his director's instruction) when the actress playing the role became violently ill just before the show.

Also memorable was the time he subbed at the last minute for Lucentio in Taming of the Shrew (he was already in the show, playing Christopher Sly). But he couldn't read the script, so he called out for some reading glasses. "About nine people ran toward the stage," he says. He selected a pair of readers and there was a huge round of applause.

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When things go sideways in such spectacular fashion, there's no calling "cut" and reshooting as you would in moviemaking – and this aspect of the live theatre experience seems to appeal to the audience. Theatregoers, according to these artistic directors, rarely complain or ask for refunds.

"It's like they're insiders at some special happening, and the rest of the world can't be part of it," says McPhee. Audiences, he adds, tend to get it. "With the reality of the resources that you have at those very last minutes, what can you do? The show must go on."