“What am I doing right now?” Sara Bareilles has just blustered into a German beer bar on New York’s Lower East Side, lugging a giant duffel bag. “I’m so frazzled!” she apologises. “I just got back from the book tour. And for the play, we’re in re-­examination mode.”

The book is Sounds Like Me, her just-released, memoir-ish collection of essays that’s a New York Times best-seller. The play is Waitress, a musical opening on Broadway at the end of April, for which Bareilles is a first-time composer-lyricist. “What’s Inside,” an album of Waitress songs sung by Bareilles, just arrived on Epic, and the plaintive single, “She Used to Be Mine,” is climbing Billboard’s Adult Top 40 chart. But tonight, it’s the show that’s most on her mind. “We’re going back through the script, re-addressing some musical moments. The next big event will be a workshop in December. We’re not quite starting from scratch – because then I’d gouge my eyes out.”

“I’m not a showstopper,” she admits. But her songs – lush ballads like “Gravity” and ­inspirational anthems like “Brave” – have commercial clout to rate with any super-celeb. Her last three albums reached the Billboard 200’s top 10 and have sold more than 2 ­million copies ­combined in the United States, according to Nielsen Music. Her first single, 2007’s “Love Song,” hit the Billboard Hot 100’s top five; 2013’s “Brave” sold 2.4 million downloads and soundtracked a Microsoft ad ­showcasing inspiring women like Malala Yousafzai. In between those hits, she judged on the third season of NBC’s “The Sing-Off.”

It’s a rare career niche: making mainstream hits while retaining ­creative freedom and, now, ­confidently leaping into the ­theater world. “I’ve gotten advice, and ­sometimes I’ve taken it and ­sometimes I haven’t. And somehow through the muck of it all I still really feel like myself.”

Sara Bareilles

“Because she’s, in a way, a ­mainstream artist, it’s easy for people to overlook her proficiency,” says Ben Folds, a collaborator. “You can forget she’s one of the best singers around.”

That’s a big part of what ­convinced veteran Broadway producer Barry Weissler that Bareilles should write Waitress, the story – based on the late Adrienne Shelly’s poignant 2007 film – of Jenna, a diner server trapped in an abusive marriage and with an unwanted pregnancy. “We tried other writers, and they couldn’t tell the simple, heartfelt story. This is not a sprawling musical with big sets and big chorus numbers. It’s lean and mean.” That suits Sara fine. “I would have had a really hard time writing a big ­musical. I don’t know what that would look or sound like. Probably not very good!”

Her self-awareness and unshowy instincts have defined Bareilles’ pop career. It helped, she says, that she was 27 when her first record came out. “I don’t know that I would have survived this industry if I had entered it at 16 or 17. So many aspects of it are toxic to the human condition. Without enough belief in oneself, I can easily see why you’d make decisions that in 60 years make you say, ‘I really didn’t want to do that.'”

Sticking to her guns has meant ­winning battles over her hits – on “Love Song,” she was encouraged not to play piano (she did) and a label executive complained that “Gravity” didn’t have enough ­choruses or drums (it stayed as is, and it’s now one of Bareilles’ most-requested songs). “She’s a real person and she’s an adult,” says actress Rashida Jones, who got her first directing gig when Bareilles asked her to helm the video for “Brave.” “There’s something really true about her songwriting, her voice, her face – nothing about her feels pushed or manufactured.”

Though Bareilles looks every inch the New Yorker today in head-to-toe black, she’s a California girl who grew up among the ­redwoods in the northern coastal town of Eureka. She fell in love with theater, and theater folk, early on. “I have a lot of baggage, which I have to process and negotiate on a daily basis,” she says. Being told to wear hair extensions and short sequined dresses on camera – while it wasn’t, she acknowledges, a ­terrible burden – “triggered a lot of that stuff. Like what I have to offer as a ­mouthpiece and a mind is not enough. It made me want to rage on behalf of all those girls who feel like they’re being asked to be something they don’t want to be in order to fill in a blank.”

Bareilles has been listening to “1989” on repeat and says that Taylor Swift, a friend and supporter, once made her a somewhat random but very generous offer: “When my bus broke down, she contacted me and said, ‘Let me help you get a f—ing bus!’ ” And in 2012, when Katy Perry felt the wrath of the Internet for the similarities to “Brave” some heard in Perry’s hit “Roar,” Bareilles took the high road, saying, “There’s better shit to do than worry about that,” and noting that the controversy ­probably helped her record sales. “Katy and I have known each other for a really long time,” says Bareilles today. Still, she admits, “I have an odd relationship with my ­contemporaries. I don’t see myself in them, and I think we feel very much in separate worlds.” Three years ago, after 14 years in Los Angeles and a “f— it, let’s go get drunk in Brooklyn for a month” trip with her sister, Bareilles moved to New York. She’s single and lives in Nolita. She auditioned for the part of Cinderella in the 2012 Shakespeare in the Park production of Into the Woods – a role she lost to Jessie Mueller, who’s now the star of Waitress. “That bitch,” says Bareilles with a giggle.

Writing a musical was an idea she previously had only “fooled around with,” specifically with her close friend Jennifer Nettles of Sugarland. “It was called Lesbians. It was about a women’s college in the ’90s, like during Lilith Fair. We wrote five songs for it! We could totally do an EP.” When ­director Diane Paulus asked her to join the Waitress team, the induction into the theatre scene “felt so official,” says Bareilles. “She just believed I could do it.”

As a show that, like Bareilles herself, is not brazenly commercial, Waitress is a gamble for big-ticket Broadway, and the artist certainly isn’t ­abandoning pop music; she started working with Brandon Creed, Bruno Mars’ manager, a year-and-a-half ago. But in the New York theatre ­community, Bareilles may well have found the ideal next stop on her meandering career path – another seemingly niche project with the potential for mass appeal.

“It has been so refreshing to me. The music industry can sadly be very competitive. There’s not as much of that air of camaraderie, and I’ve been so delighted by that in the theatre community. And they f—ing work harder than any of us!”