Pouring herself a cup of herbal tea at a downtown restaurant, Sara Bareilles is the picture of serene composure. You’d never guess that she’s been working like a maniac for the better part of two years. She is on the verge of releasing “What’s Inside: Songs From Waitress” on 6 November, featuring songs from the new musical she has written, which due to open on Broadway on 25 March.

“It was an act of self-indulgence. I couldn’t quite pass off the show without getting a chance to sing these songs myself.”

Sara was first approached about the musical, and writing a book, “Sounds Like Me: My Life (So Far),” during the same period — ironically, while taking a “mini-hiatus” from recording and touring. “I was hanging out with my sister,” she recalls, when she was contacted by her literary agent and by Diane Paulus, the Tony Award-winning director of recent Broadway revivals of Pippin and Hair, regarding Waitress.

Though a fan of musicals since childhood, with favorites ranging from Oklahoma! to Chess, Sara had never seen the film Waitress before. Watching it, she was immediately attracted to the heroine, Jenna (played by Keri Russell), an inventive pie maker trapped in an abusive marriage.

“I really loved how flawed and messy Jenna is. I liked that she was a soulful, creative person trying to find an authentic way to express herself. I found so much natural similarity between the character and myself that it wasn’t a stretch to find my way in. She’s someone who has a great capacity for love, but feels very broken sometimes.”

Sara had first seen the actress who would play Jenna on stage, Jessie Mueller, in Mueller’s Tony-winning performance in the title role of Beautiful: The Carole King Musical. “She was so grounded and graceful, and just had so much heart.”

Both Waitress’s leading lady and its composer/lyricist earned good notices in Cambridge, though Sara hasn’t read them. “I don’t really read reviews. My theory is that the good ones inflate your ego, and the bad ones can just ruin you. I’ve found it’s more productive to stay in the room with what I’ve created, rather than worry about how it’s being perceived.”

Working on Waitress made Sara realise how much she herself has grown since her 20s. “I’m about to have my first Broadway musical. When I made my first album, I was so overwhelmed that I don’t think I was able to enjoy it. It’s nice to be in a place where I can create something new and realise how precious it is.”