Rachel Webb, of Broadway's '& Juliet,' knows the show must go on. Some days, she’s why it does.

Rachel Webb, the “first cover” for Juliet in “& Juliet."

Actor Rachel Webb was selling Nikes at a store on the Upper East Side, about a year ago when her agent called to say she’d gotten not one, but two roles in a new, hit musical from the West End, called “& Juliet.”

On Tuesday, “& Juliet” was nominated for nine Tony awards, including “Best Musical” and “Best Performance by an Actress in a Leading Role in a Musical.”

But little of that glory goes to Webb directly. She’s an understudy, and despite putting in all the hours of a lead, she receives less of the adulation – and sometimes, she gets just the opposite.

Once, when the show was running in Toronto, the lead was too sick to finish the performance. Webb was backstage when the stage manager announced that she'd be taking over as Juliet mid-show. The audience groaned.

“We were just like, well, they hate me,” she said, recalling the moment in a spacious lounge at the Stephen Sondheim Theatre in Midtown.

The tricky thing about being an understudy is that audiences have spent a lot of money, navigated the hordes of Times Square and crammed themselves into tiny seats — and they didn’t necessarily do that to see you.

But what was the alternative, Webb wondered. She sounded bubbly and upbeat imagining the scenario: Give everyone a half-refund? Didn’t they want the show to go on?

Webb, center, as Juliet.

That day in Toronto, she didn’t have time to worry about how the audience felt. She was due onstage in minutes and had to change out of her ensemble costume and into Juliet’s blue jumpsuit.

She said the crowd, despite their reticence, soon warmed up to her, and laughed at her jokes.

“Well, not my jokes,” she added, “but the script.”

The script, by Schitt’s Creek writer David West Read, is a girl-power take on Shakespeare’s “Romeo & Juliet,” imagining Juliet’s life if she didn’t die after discovering Romeo’s dead body.

Chances are, you already know the songs in “& Juliet” — they are some of the greatest hits of the '90s and the aughts, from Britney Spears, Kelly Clarkson and the Backstreet Boys — written by Swedish superstar producer and songwriter Max Martin. The dancing is inspired by hip-hop, with a whiff of TikTok, and it’s all brought to roaring life by a young and diverse cast.

The New York Times' chief theater critic began his review practically apologizing for what he was about to do, which was to endorse a jukebox musical: “I liked it. It felt so wrong; it felt so right,” he wrote, quoting a Katy Perry line from the show.

Webb, 25, plays Judith in the ensemble, and she’s the “first cover” for Juliet, meaning she plays Juliet if the main actress, Lorna Courtney, can’t make it. There are two other covers after Webb. The show also has swings, who go on for members of the ensemble.

A spokesperson for the Actors’ Equity Association said that, of the 33 shows currently on Broadway, 448 people have “contracted understudy, stand-by, full swing or partial swing assignments.” She said it is "very common" for chorus performers to be understudies, though the association does not track how often they go on for the principals.

“Understudies and swings are the heroes of the theater,” said the show’s choreographer, Jennifer Weber, who has worked with Webb in both of her roles: Judith in the ensemble, and Juliet.

Weber said that most people probably don’t realize that when you are an understudy, “you spend most of the rehearsal process focusing on your main track.”

Webb could sit in on rehearsals with Lorna Courtney to observe her playing Juliet. But what she couldn’t do was see how her own delivery landed with other actors, or build muscle memory in the role; Webb said it required “a wide imagination.”

“What I had to do, essentially, was go home and do my own homework,” Webb said. She recorded herself playing the lead on her iPhone’s voice memos, which she would then study on her own.

The first time Webb played Juliet, she’d received around 24 hours’ notice that she’d be subbing in. Her friend Veronica, the second cover for Juliet, ran lines with her. The production held an emergency rehearsal for Webb before the show, mostly to help her learn things like where to stand so she didn’t fall through a trapdoor.

“I literally had to learn my safeties so that I can get into the harness correctly and go up on the chandelier, 20 feet in the air, in front of a thousand people,” she said.

When the show was over, she cried with relief during her last song, “Roar.”

“I don’t even remember the whole show,” Webb said. “All I remember was going up on the balcony at the end with the microphone in your hand and the sequins and just sobbing.”

That moment was the culmination of a long journey.

Webb was raised by a single mom in Dallas, Texas, the youngest of four kids. She said that as she got older, her family struggled to stay afloat financially, and lost their house to foreclosure. Webb's mother, an entrepreneur who supported her ambitions and whom Webb describes as someone who “knew how to make dreams come true” died in April 2022, just a few weeks after Webb was cast in “& Juliet.”

Webb didn’t know much about Broadway when she was growing up, but starred in local shows about Whitney Houston and Michael Jackson.

In 2015, when she was in high school, she went to New York City with a group of six performers called Young Gifted and Black Artists, led by her mentor, Curtis King. They saw “On the Town" with Tony Yazbeck — it changed her life.

“In Dallas, I hadn’t seen anything as polished as what I’d seen on that Broadway stage,” she said.

From that day on, she said she took performing “seriously.” She earned scholarships to complete her degree in musical theater at Texas State University. And then, two years after graduating, she made her Broadway debut.

In February, her mentor, Curtis King, saw her on Broadway as Juliet: “I cried, he cried.”

She said the question most non-musical theater people want to know is: How do you keep two roles straight in your head?

That, she said, is not easy. There’s a moment when one of her characters responds to the other, and it all starts to feel very “meta.” She didn’t want to get into specifics to avoid spoilers.

“I have to really, really compartmentalize in order to not fall apart in those scenes,” she said.

Webb said that she’s grateful that within her show, there is “no drama” around the word “understudy” or “cover.”

“It can be so healing and so beautiful to say, 'hey friend, you're up. Tag team,'" she said.