Everyone Knows Sutton Foster Can Sing. Now We Know She Can Juggle.

There’s busy, and then there’s bonkers.

Sutton Foster, one of musical theater’s most celebrated performers, had already committed to starring in a City Center production of “Once Upon a Mattress,” on top of developing concert shows for Carnegie Hall and Café Carlyle, when she was approached last fall about stepping into the lead female role in the Broadway revival of “Sweeney Todd.”

She hesitated.

“Sweeney” wanted new stars in January, the same month as the “Mattress” production. She would have to simultaneously master two scores and two stagings while building the bespoke concert shows and learning to speak with a Cockney accent. And even if, as it turned out, “Sweeney” was willing to wait until her “Mattress” run ended, she’d still have to do double duty — rehearsing “Sweeney” during the day while performing “Mattress” at night.

She said yes.

On Feb. 9, she took her first bows as Mrs. Lovett, the shamelessly resourceful pie shop owner in “Sweeney Todd,” alongside Aaron Tveit, also in his first night, as the bloodthirsty barber (the show’s title character). It was just five days after she took her final bows as Princess Winnifred the Woebegone, a coarse but determined marriage candidate in “Once Upon a Mattress,” and the applause was thunderous.

So how did she do it?

Plenty of people hold down two jobs at once. There are repertory companies in which actors perform in a rotating selection of shows. There are Broadway stars who spend offstage hours filming television shows. And 40 years ago, Cynthia Nixon, while still a teenager, spent three months performing in two Broadway plays at the same time (dashing from one theater to another and back again).

But still, learning the starring roles in two vocally and physically demanding musicals at virtually the same time is a feat. And, inevitably, Foster faced hurdles. Among them: Halfway through “Mattress” rehearsals, she came down with Covid (for a third time).

In a series of interviews, Foster talked about why she agreed to the potential pileup and how she approached the work.

“I do love a challenge,” she said.

She has always revered “Sweeney Todd.” The commitment being sought was relatively short — 12 weeks. The two productions were willing to coordinate and adapt to accommodate her availability. And another factor, even for someone of her stature, was the ever-present awareness of the vagaries of theater.

“It’s an embarrassment of riches, and I’m very aware of how lucky I am,” Foster said. “I get these opportunities to play roles like this, and I don’t know how long that will be, you know? I know how rare it is. And I also know it’s fleeting.”

She decided to study “Sweeney” first, even though she was starting performances in it last, because of the limited time between shows. “The score of ‘Sweeney’ is essentially an opera,” she said. “It’s so complex, and one of the hardest scores I’ve ever had to learn.”

So she would spend as much time as possible on “Sweeney” in November and December with the goal of being able to run through that show, without an audience, by early January; then she would set “Sweeney” aside to learn “Mattress.” Once “Mattress” was up and running in late January, she would simultaneously resume rehearsing “Sweeney.” The two-week “Mattress” run would end on Feb. 4, which would give her a little less than a week to finish preparing for “Sweeney.”

“To me it’s clear that a normal human could not possibly do that,” said Lear deBessonet, who directed Foster in “Mattress.” “It’s completely epic.”

Her time was further constrained by the concerts. She needed to pick and learn the songs and patter for the November Carnegie Hall show, which she was developing with a fellow Broadway star, Kelli O’Hara, as a homage to Julie Andrews and Carol Burnett’s 1962 Carnegie concert. And then she had to create a new holiday show for a seven-night December engagement at the Café Carlyle.

“It was major compartmentalizing in my brain,” she said.

Foster, 48, has been a darling of Broadway since 2002, when she broke through, and won her first Tony Award, as a tap-dancing, husband-hunting flapper in “Thoroughly Modern Millie.” Her performances are characterized by high energy, irrepressible spirit, and go-for-broke physical comedy. She can sing (“the voice of a trumpet,” this paper once wrote), she can dance (“She’s the Top,” read a headline in Dance Magazine), and she can hold the stage.

“She has that thing you can’t teach,” said Thomas Kail, the “Sweeney” director. “As they say in ‘Gypsy,’ ‘You either got it, or you ain’t,’ and she’s got it.”

Over the years, Foster has performed in a dozen Broadway musicals, starring as the nightclub singer Reno Sweeney in “Anything Goes,” the ogress Fiona in “Shrek,” a disfigured woman seeking faith healing in “Violet,” and, most recently, as Marian, the librarian, in “The Music Man.” She has been nominated for Tony Awards seven times, and has won twice. She took a seven-year break from Broadway while appearing in the television series “Younger”; some fans also know her for the short-lived television drama “Bunheads.”

The “Mattress” and “Sweeney” roles lead with comedy, and Foster knows how to land a laugh. She’s also uninhibited — both characters, at least in her renderings, involve spitting up food onstage. “I guess it’s now my thing,” she said.

But the shows are also quite different, tonally and vocally.

“Once Upon a Mattress,” adapted from “The Princess and the Pea” and featuring music by Mary Rodgers, is built for a belter — Foster’s sweet spot — and calls for a madcap performance by the leading lady, culminating in a virtuosic display of squirming as the princess finds herself unable to fall asleep. “Sweeney Todd,” with a score by Stephen Sondheim, is more complex — tongue-twisting lyrics and unexpected notes — and, given its gruesome plot involving both homicide and cannibalism, much darker. Both characters are looking to upgrade their circumstances, but whereas Winnifred is blithely confident, Lovett is daffy and desperate.

The roles require a very different sound. “If I were to talk about it technically,” Foster said, Lovett “sits right on my break” — the place where the parts of her singing range meet. “So it’s constantly swapping back and forth, between my chest voice, my head voice, my mixed, and all in there and it’s like constantly navigating, and negotiating. It fits the character.” Winnifred sits “lower and brassier and shinier,” she added, but that feels right too. “It fits the character that she would be WAAH!”

Her first task was dialect. Foster, who spent her early childhood in Georgia, has occasionally performed with a Southern accent, but Lovett’s Cockney accent was new to her, so she began working with a dialect coach, learning to open her vowels and drop some of her consonants. “I was really nervous — afraid to make a sound, thinking, ‘I’m going to offend every British person,’” she said. At one point, she took her daughter to see “Wonka,” and found herself staring at Olivia Colman’s mouth, trying to learn from the English actress’s facial movements. “At first it was so technical,” Foster said. “I couldn’t get past the dialect to get to the words.”

At the same time, she began studying the score. Alex Lacamoire, the show’s musical supervisor, would play the notes she needed to sing; she would record parts of their sessions. “I listen over and over,” she said, “so I can nail it into my brain and let it seep into my bones.”

The “Sweeney” revival began a year ago, with Josh Groban and Annaleigh Ashford in the starring roles, and is a hit; after three months with Foster and Tveit, the show is expected to end its run. Foster had seen the revival twice, but opted not to listen to cast recordings or watch the movie as she prepared to join the cast, saying “I’m afraid of getting either intimidated or someone else’s interpretation stuck in my head.”

Once the Carlyle concerts were behind her, she began rehearsing with Tveit. Other than a bit of time off for the holidays, they worked steadily through December, so that by Jan. 5 they could do a so-called put-in rehearsal — a run-through of the show with full company and full orchestra, but only the new members of the cast in costume. “I was really proud of myself that I was able to get through the show, and I was really grateful that I wasn’t going into the show that night,” Foster said. “I wasn’t like, ‘I’m never going to be able to do this.’ It was more, ‘I’ve done it, and I have so much more I want to figure out.’”

But that figuring out would have to wait.

“I had to let it go,” she said. “It just had to be put on the back burner, and I made ‘Mattress’ my focus.”

“Mattress” was being staged as part of City Center’s Encores! series, which presents short-run productions of classic musicals, and has a limited rehearsal process. Foster knew none of the songs when she arrived, so she spent two days huddled with that show’s creative team before joining the rest of the cast.

“It was a little scary, in the sense of, is my brain going to be able to handle this? Do I have enough juice left in the tank, even creatively? Will I crash and burn?” she said. “But that’s also part of taking on any challenge. Parts of me are really excited, parts of me are calm, parts of me are nervous. It’s all the things.”

She had learned half the show, and made it through half the rehearsal days — when she tested positive for Covid, forcing her to take several days off. And when she returned she had to rehearse in a mask, even while singing.

“I was determined to keep going,” she said.

“Mattress” opened Jan. 24 to enthusiastic crowds and strong reviews. “Foster’s glee in taking possession of the stage,” the critic Elisabeth Vincentelli wrote in The New York Times, “creates an all-encompassing manic energy that both the audience and her scene partners feed off.”

With “Mattress” on its feet, Foster resumed rehearsing “Sweeney,” learning Lovett during the day while performing as Winnifred at night.

“I was witnessing her flip back and forth like she was changing channels on a TV,” said Mary-Mitchell Campbell, the “Mattress” musical director. “It was insane but phenomenal.”

It was also painful. “‘Mattress’ was way more physical than I was anticipating,” Foster said. “I’d go to sleep and my hip would hurt, my shoulder, my back, my foot. Winnifred beat me up.”

“Sweeney” was no joke either — she was developing physical antics that included massaging Tveit’s chest with her feet while she sang upside down.

The pressures on her body are harder than they might have once been. Foster thinks of performers as athletes — she is hyper-focused on sleep and hydration; she naps and meditates and meets with a vocal coach and a life coach; she teaches dance cardio and she crafts to stay sane. Age, she said, has been a mixed blessing.

“Physically, it gets harder,” she said. “But I’m also a little bit wiser, and I have more ability to ask for help — to figure out what I need to protect myself, and to modify. I still get nervous, but I have more tools to handle it than when I was younger. And it’s easier because I have so many things in my life” — including her 6-year-old daughter — “that are just as important as what I do for a living.”

Ten days into the “Mattress” run, she arrived at the Lunt-Fontanne Theater for an early afternoon “Sweeney” rehearsal and immediately began scarfing down a peanut butter sandwich, saying “I need some calories” as she ate her first meal of the day.

As she ran through a scene with John Rapson, playing the Beadle, she missed a few lines, and looked in the wrong direction for an entrance by Tveit. “Did I say most of the words right?” she asked of no one in particular. Kail, the director, clambered onto the stage and provided some suggestions about how to think about her character’s motivation. They ran the scene again. “Better?” she asked, wiping her brow.

During breaks, she drank from her daily 32-ounce blend of greens, electrolytes and collagen, glanced at her phone, told her castmates a story about her daughter, and marveled at the hulking industrial set. She was relentlessly upbeat, at times snorting with laughter. Back onstage, as she worked through scene after scene, there were minor stumbles — at one point, she collided with an ensemble member while exiting the stage — but also confirmation of her comedic choices, like wiping her nose with pie dough, which would develop into a crowd pleaser.

She said parts of “Sweeney” had slipped away during “Mattress,” but were still there, somewhere. “It’s like in that dark corner of my brain,” she said.

When she returned to City Center in Midtown for a “Mattress” performance, among those in the audience was Tveit, marveling at the Winnifred-Lovett shuffle. “We had run ‘Sweeney Todd’ in the afternoon, and I went home and had dinner and somehow saw her two hours later in ‘Mattress,’ and just thought in my brain, ‘What is happening?!’” he said. “I couldn’t imagine doing that myself.”

For Foster, there was one more weekend of “Mattress” performances, and then a few days of full “Sweeney” focus before facing an audience. On the day she and Tveit were to finally unveil their performances, she arrived early for a photo shoot, gathered in a circle with her new cast, obsessively ran through the wordplay-dense Act One finale, “A Little Priest” (“I’m the crazy lady talking to herself”), and, while putting on her wig and makeup, had a video call with her daughter, who was eating a lasagna Foster had made that morning.

The crowd was electric — thrilled to see Tveit, thrilled to see Foster, thrilled to see Joe Locke, the “Heartstopper” star who had joined the cast 10 days earlier. Foster wasn’t sure how to process the raucous enthusiasm — “there’s a little bit of disassociation that happens,” she said — but by the next morning, about 12 hours after leaving the theater, she finally took a breath.

“I still feel a little dazed — really tired, really proud, really relieved,” she said. “Yes, I flubbed a couple of lyrics, but my greatest fear is that I will get off and I won’t be able to get back on — that it will all fall apart — so what I was grateful for is that I got right back on, and I thought, that’s such a good find, for my own peace of mind. Remember that: I’m not going to derail the train.”

That first performance, she said, was only another beginning. She has three months now to grow in the role, and then, she hopes, a vacation, followed by a season of concertizing.

“I like to think of myself not only as an actor, but also a detective, and as I’m playing the role I’m also paying attention to what’s working, and what isn’t,” she said. “You get an opportunity every day to keep discovering. And I don’t think you ever stop.”