Stepping into the lead role of a smash Broadway musical is a daunting prospect for anyone – but for an actor with no previous experience on the Great White Way, it’s got to be terrifying. Fortunately, the actress who just took top billing in Broadway’s Waitress was already one of the show’s key ingredients – after all, she single-handedly wrote the play’s music and lyrics.

Even so, Sara Bareilles was oscillating between anxiety and exuberance when Billboard spoke to her just ahead of her big Broadway debut as Jenna, a role she inherited from the beloved Jessie Mueller. “I am incredibly nervous, but my base-line feeling is joy and gratitude and little-kid excitement.”

On her first night, she helped the Brooks Atkinson Theatre shatter its record for highest-grossing single performance with a hefty $180,000 box office total.

Not bad for someone who hadn’t acted since high school — at least, if you believe Bareilles.

“I don’t believe she’s not an actress,” says co-star Will Swenson – who recently stepped into the role of Jenna’s husband, Earl – with a conspiratorial smile. “I think she’s been suppressing the desperate desire to do musicals and this pesky pop stardom overtook this musical actress’s career. It’s just been a big distraction for her.”

He’s joking, but Swenson might not be entirely wrong. While Bareilles says acting in Waitress was never the end goal, she admits the urge to act on Broadway “must have been slightly subconscious” when she signed on to the project. So when she learned acclaimed lead Mueller was moving on, “Something spoke to me. I had a long conversation with Jessie Nelson, our book writer, and our whole creative team, and it just felt like the right time to come in.

“I love that I get to come into a healthy show; we’re not doing everything we can to keep the doors open,” Bareilles says. Of course, joining a hit production can be daunting for different reasons. After watching Mueller “bring so much honesty and vulnerability” to Jenna for a year, Bareilles admits she was “completely intimidated” stepping into a role originated by a Tony winner. “Jessie has been such a gift to the show – she’s a really creative, funny, smart actor who happens to also sing her ass off – and now the trick is surrendering to the fact that I have to find my Jenna. That’s the nature of the beast – we are separate people so I have to find my own character.”

“If I hadn’t known this was her first professional acting gig ahead of time, I wouldn’t have guessed it,” Chris Diamantopoulos. the cast’s new Dr. Pommater, tells Billboard. “I don’t think you can be that gifted of a poet and storyteller in song and not be able to tap into what it takes to act a role…. She’s just this unicorn. People like her don’t exist.”

“She’s a total natural. She’s in no way a novice,” Swenson echoes – although he admits that his perspective isn’t exactly unbiased. “I don’t think she knows this yet, but I’m a crazy fan. I’ve been incredibly awkward around her in rehearsals and I’m trying not to seem like a stalker, but I could sing you every one of her songs right now.”

“What Broadway people relate to with Sara is her songs aren’t just pretty melodies – they tell stories. It made sense on paper that she would right out the gate write a hit musical,” Diamantopoulos says.

“She writes emotional songs and that’s what theater people are drawn to – the catharsis of singing a chest-open song,” Swenson says. “She puts her heart out there and writes raw, emotional songs, and that’s the goal of a musical theater song as well. Maybe we feel her pop songs were musical theatre songs in a way to begin with.”

After tasting Broadway success both behind the scenes and on stage, it’s hard to imagine this will be Bareilles’ last foray into musical theater. When Billboard asks if she’ll write another Broadway musical, Bareilles simply answers, “My pat answer is ‘never say never.’ I pride myself in that I make decisions on what happens next in my career from an organic place, and I don’t know what that may look like now. I love the idea of writing another musical, I love the idea of being a part of another cast at some point. I love the idea of exploring television or film acting. The gift this show has given me is the curiosity of exploring new frontiers. I didn’t know I’d ever write a Broadway musical. It’s exciting to try something new on and enjoy what it feels like to be a student again.”

Sara Bareilles as Jenna