As Omicron spreads, shows are relying on replacement actors more than ever. And productions without enough of them have had to cancel performances.One evening in November, just a few hours before showtime, stage management told LaQuet Sharnell Pringle to prepare. A practiced swing, Pringle covers the female parts in the ensemble of the new Broadway musical “Mrs. Doubtfire.” She also understudies the role of Wanda, the social worker usually played by Charity Angél Dawson.The musical was still in previews. Pringle had never really rehearsed as Wanda. But she had studied the script, mastered the choreography and watched dozens of performances. So when Dawson called out (for a reason unrelated to Covid-19), Pringle went on.“It’s the job,” she later explained. “It’s the gig — to be able to be thrown on in a moment’s notice and to be able to deliver.”Swings and understudies are the undersung heroes of Broadway theater. (Off Broadway and regional theater productions may or may not hire them, depending on a production’s budget and priorities.) If a curtain rises when one or more actors has suffered illness or injury, that’s because a swing or an understudy has stepped in, sometimes with just a few minutes to get into costume, sometimes in a costume that isn’t even theirs. At a time of pandemic uncertainty, their contributions have become even more essential.For those unfamiliar with the terminology: Understudies can fill in for one or more of a play or musical’s principal characters. They may regularly appear onstage in a smaller role or they may spend most nights backstage, performing only if needed. Swings have no regular role in a show. Instead they cover up to a dozen ensemble parts in a musical, each with its separate vocal and dance track. Swings may also cover a principal character or two. (Some shows also use alternates, who take over principal roles for a number of performances on a predetermined basis.)In the past week, about half of the shows on Broadway canceled some number of performances. Charlotte St. Martin, the president of the trade association the Broadway League, seemed to blame certain understudies for these cancellations in an interview published Monday in The Hollywood Reporter. “My educated guess is the newer shows maybe have understudies that aren’t as efficient in delivering the role as the lead is,” she said.St. Martin quickly apologized, but not before fans and stars had leapt — or maybe, grand jetéed — to the defense of swings and understudies. “*My* educated guess is that when employers consistently reject our efforts to negotiate for more swings, understudies and sub stage managers, because the industry model has grown dependent on people working sick/injured, it’s short sighted and unsafe,” Kate Shindle, the president of Actors’ Equity, wrote on Twitter.In an interview a few hours later she expanded on those comments. “I hope that Broadway producers will be as proactive as possible in making sure that there is adequate coverage,” Shindle said. “Covid has changed how that looks.” Shows that don’t have enough coverage are forced to cancel.Hiring practices aren’t the only conventions worth re-examining. Historically, swings and understudies have not typically received dedicated rehearsal time until after a show’s opening night. Thursday night, after the fourth preview of “The Music Man,” Hugh Jackman saluted the understudy Kathy Voytko for going on for an absent Sutton Foster, despite having first rehearsed the part of Marian Paroo earlier that day.Without their own rehearsal schedule, swings and understudies have had to learn scenes, songs and choreography by observation and osmosis. (And in reverse, as they were often relegated to watching from the house.) If they had to go on in previews, like Voytko did, their castmates knew to “shove with love,” nudging them toward their correct positions.But the pandemic has encouraged a reckoning with the “show must go on” culture. Graham Bowen, a longtime swing for “The Book of Mormon,” described the mood backstage as, “Hey, if you’re just not feeling great, don’t come to work. It’s OK. We got this.” He referred to nights when only the Playbill-listed cast performs as “unicorns now.” (In mid-December, Bowen tested positive for Covid-19 and had to quarantine, so other swings, many of whom he has trained, are performing in his place.)In response, a few shows are rehearsing understudies earlier, sometimes right alongside the main cast or with the principal director rather than an assistant. “I told them from the jump, ‘We have to all trust each other because there’s all this intimate work. You can’t just come in halfway through,’” said Robert O’Hara, the director of “Slave Play,” which includes several sex scenes. And there is also a drive to allow some understudies to deliver distinct performances, rather than simply copying the work of listed performers.Rehearsal makes some aspects of the job easier. But it doesn’t alleviate the anxiety of not knowing whether you will go on. Or the weirdness of earning a Broadway salary when you may not set foot on a Broadway stage for weeks at a time. Or the feeling that you may be disappointing ticket holders if they see your name printed on a Playbill slip.Despite her longtime career as a swing for “The Phantom of the Opera,” Janet Saia said she once returned a ticket when she arrived at “The Producers” and found that Nathan Lane was out. “That show is all about him!” she said.Yet understudies and swings are always hired because they can do the role — or the many roles — with the virtuosity that Broadway demands.“We’re the last resort,” said Reynaldo Piniella, an understudy on “Trouble in Mind.” “But I do know, watching my understudies rehearse, these people are incredible artists.”On Wednesday night, about 10 minutes before curtain, Piniella got his own call. Though he had already booked a last-minute job on a different Broadway show, “Thoughts of a Colored Man,” he hurried to the “Trouble in Mind” theater, stepped into a costume he had never worn and made his entrance. “I bumped into pretty much every piece of furniture on that stage,” he said. “But I was prepared to tell the story.”In late November and early December, a few weeks before the current string of cancellations, I interviewed a number of swings and understudies about the rewards, stresses and peculiarities of the job and how Covid-19 has altered waiting in the wings. These are edited excerpts from the conversations.Galen J. Williams of “Slave Play” started rehearsals much earlier than usual for an understudy, partly because the play contains quite a bit of intimate work.George Etheredge for The New York TimesGalen J. WilliamsUnderstudy, “Slave Play”What makes you take an understudy job?I’m an Aries, we thrive on spontaneity. It keeps the job exciting, never really knowing when you have to turn the light switch on. Every time you go on, it feels like it could be the last time. So let me really give it my all, go full in and have a good time.Andrea SyglowskiUnderstudy, “Pass Over”What was your rehearsal process like?From the first week, we rehearsed with the main cast, warmed up with them. We were never under any obligation to do exactly what the main cast was doing. We were actually encouraged to create our own characters.The show ended its run before you went on. How did that feel?It was really upsetting. It became increasingly possible that I was going to go on at the end of the run. So the anxiety and the tension increased. I was so excited to be a part of the first Broadway play back. I wanted to be able to show my work. I’m sure every understudy feels that way.“My fantasy is that the actor I’m understudying books some big Marvel movie,” said Reynaldo Piniella, an understudy on “Trouble in Mind.” George Etheredge for The New York TimesReynaldo PiniellaUnderstudy, “Trouble in Mind”How do you feel about understudying?I equate it to being the sixth man on a basketball team. You’re not a starter, but you understand that you play a very vital role. When your number is called, you want to make sure you’re on your game. My fantasy is that the actor I’m understudying books some big Marvel movie. This show is my Broadway debut, and I would love to get on that stage at least one time, feel what that’s like.LaQuet Sharnell PringleSwing, “Mrs. Doubtfire”How do you keep all of those roles in your body and head?I physically need to do choreography every single day, I physically need to say words out loud every day so that it’s a part of my muscles. I will do cross training to make sure I have the stamina to do all of the dance choreography. I increase my voice work so that I am strong enough to sing the songs no matter what. I generally read the script two to three times a week.If you’re not on, do you ever relax backstage?It makes me feel weird if I’m just chilling out. As I’m watching the show, I try to say the lines in real time. I’m trying to go over steps that I want to perfect and really understand. I wait until I get home and I’m in my Epsom salts bath to relax.Graham BowenSwing and co-dance captain, “The Book of Mormon”How does it feel to receive that last-minute call?I have experienced the stage manager coming into a dressing room and saying, “Hey, we need you for the next scene.” And that scene would be in a matter of minutes. It’s kind of one of the best parts of it. You have this adrenaline rush. It’s definitely nerve-racking. But you can ride that and really have a wonderful experience.Janet Saia outside the Majestic Theater, where she’s an understudy in “The Phantom of the Opera.” “You make mistakes, but then you say to yourself, ‘I’m covering eight roles, give me a break,’” she said.George Etheredge for The New York TimesJanet SaiaSwing, “The Phantom of the Opera”What makes a great swing?As a swing, you have to have the fantastic five. First, you have to be fast, you have to learn things fast, you have to adjust to change fast. Second, you have to be fabulous at what you do. The next one is focused, really, really focused. And flexible. And then the last one is fastidious, you’ve got to pay attention to all the details and make sure they’re all in there.With so many roles to cover, have you ever mixed them up?There’s a number in “Phantom,” “Don Juan.” It has all the people I cover in the ensemble around a table with props. One time I went out there and I had the jug and everything. Two seconds before the curtain goes up, somebody came in and said, “That’s my jug.” Then I realized, I’m not this part. Instantly, I had to switch my brain and go into the correct part. You make mistakes, but then you say to yourself, “I’m covering eight roles, give me a break.”Cameron Adams is an ensemble member and understudy in “Mrs. Doubtfire,” which recently resumed performances after a 10-day hiatus prompted by coronavirus cases.George Etheredge for The New York TimesCameron AdamsEnsemble member and understudy, “Mrs. Doubtfire”Have you ever felt that you were disappointing an audience?I covered Kelli O’Hara in “Nice Work if You Can Get It.” Her name is above the title, people are excited to come and see Kelli O’Hara. I understand, because I want to see Kelli! I don’t think I’ve ever come off feeling, like, “Oh, my God, they hated me.” But I sometimes think, “Why do I do this to myself?” Because I get so nervous every time. That doesn’t go away.Sid Solomon of “The Play That Goes Wrong” said he has “an even greater personal sense of pride of showing up to the theater every night.”George Etheredge for The New York TimesSid SolomonUnderstudy, “The Play That Goes Wrong”Has Covid-19 made you feel differently about the job?I have an even greater personal sense of pride of showing up to the theater every night, knowing just how important having understudies is, so that nobody is ever put in a position where if they’re sick, if they’re injured, if they have a family issue, they ever have to think, I’m choosing between my well-being and whether or not a show happens that evening. I’m glad to be part of a production that’s really sort of living those values right now. More