Sara Bareilles hops onto the stage at the Brooks Atkinson Theatre in the middle of Act I of “Waitress” and leaps into action.

Behind the curtain of cherry-pie pictures, there is pounding, as stagehands frantically try to get a stuck piece of scenery moving so the show can start again. In front of the curtain, Bareilles is trying to fill time by leading the audience in a singalong of “Part of Your World” from “The Little Mermaid.”

Bareilles, best known for singing her own personal pop-rockers like “Brave” and “Love Song,” is gamely moving ahead, though she’s forgetting lyrics and trying to cover the sounds of hammering.

“I started it so low that I’m not even remotely in the right key,” she tells the crowd, right before the song’s big finish. And just as she gets the final lyrics in order, the stage manager flags her down and signals that they need more time. “You need five more minutes?” Bareilles says, as she sits down on the stage, laughing. “I didn’t even finish the song, but who cares? You guys want to talk about something?”

Weeks later, Bareilles is laughing again, thinking about the experience. “Yeah, that was my Broadway debut,” she says, sarcastically. “It was kind of magical.”

In a way, though, it was. Though Bareilles is one of a growing number of pop musicians pursuing Broadway projects, it’s hard to imagine U2’s Bono and Edge, Sting or even Duncan Sheik taking the stage with no preparation to entertain a crowd at a preview during an open-ended production-related delay.

However, Bareilles didn’t think twice about it. “Yo-Yo Ma recently articulated something that I feel,” she says. “When you play for an audience you have to be a good host. They are guests in your house. That’s always been a part of how I approach my own shows.”

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The tendency to leap into situations is also how Bareilles found herself involved in “Waitress,” the first musical she has ever written.

“When Diane Paulus, our director, approached me with the idea, I was trying to be really open-minded,” Bareilles says. “I gave her a tentative yes. I didn’t know if I could write a show. I had never tried.”

Bareilles hadn’t even seen the 2007 “Waitress” movie before she first met with Paulus.

“I did start to think about the movie a lot,” she says. “What I liked was that all the characters were shades of gray. Nothing was black-and-white. There was no traditional hero. Even though you end up falling in love with Jessie [Mueller, the Tony-winning star of “Beautiful”] playing Jenna, she’s a messy character.”

Based on the “Waitress” movie — written and directed by the late Adrienne Shelly, who grew up in Jericho — “Waitress,” the musical, focuses on pie-maker Jenna and her friends at the diner as they struggle through troubled relationships.

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For Bareilles, whose pop music is packed with personal details and her own struggles, writing about other people’s issues was new.

“This project was an exercise in radical empathy,” she says. “I don’t know what it says about me, but it was really easy for me to dive into the emotional arc of another person’s story. I love digging around in that stuff. And I was really seeing myself in all of these characters so I could write songs that felt truthful.”

Paulus, who won the best director Tony for the revival of “Pippin,” has said that she recruited Bareilles for the project because of that ability. “Her music just cuts right across that proscenium and enters your gut and your heart and your mind,” Paulus told Marie Claire recently.

Bareilles says that she is thrilled with the results. “It’s the thing that I’m most proud of in my whole life,” she says. “I’ve never worked on anything this long and this sincerely. I really drank the Kool-Aid on this project.”

However, Bareilles says that she is also ready to step back from “Waitress” after it opens on Broadway Sunday, April 24.

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“I need a little bit of a break,” says Bareilles, who, in addition to working on the musical, released the album “What’s Inside: Songs From Waitress” and her memoir “Sounds Like Me: My Life (So Far) in Song” in the past year. “I need to do some laundry.”

Will there be another Bareilles Broadway performance? “Never say never,” she says. “Right now, I’m ready to step away. … I’m ready to be a big cheerleader for the show.”

She has already lined up her next project, starring as Ariel in the live concert version of “The Little Mermaid” at the Hollywood Bowl in June. And she is set to work on songs for a new album, the follow-up to 2013’s Grammy-nominated “The Blessed Unrest.” “It’s nice to feel that desire again,” she says. “I can’t wait to hear what’s calling to me next.”