As “Waitress: The Musical” opens nationwide Thursday, Sara Bareilles will not only be celebrating a concluding chapter with the Tony-nominated musical as its composer, lyricist and star.

She’ll also be celebrating her 44th birthday.

It’s been a decade-long journey for Bareilles who, as a Grammy winning singer, songwriter and pianist selling over 15 million singles, was approached to musicalize Adrienne Shelley’s 2007 hit movie of the same name.

“Waitress” is about Jenna Hunterson. Stuck in an abusive marriage and suddenly pregnant, Jenna works in a diner where she excels as a piemaker.  Her life changes with a pie contest, supportive friends and an affair with her (married) gynecologist.

The musical ran on Broadway from 2016 to 2020. A year after it opened, Bareilles debuted in the starring role. This week’s “Waitress: The Musical” arrives as filmed during a live Broadway performance.

This journey began for Bareilles with meetings with Broadway veterans about composing the score. “I knew who none of those people were. I had never seen the movie. So,” she said in a Zoom interview, “I was going in completely blind and really got my education in a totally other industry.

“Even though I grew up loving musical theater, I was a total Broadway baby. It began as this sort of experiment. I was overwhelmed by the idea; I couldn’t look at the whole project at once. That felt impossible.”

She began by watching the movie, looking for moments: “That sounded like this could be a song. Or this is a beautiful piece of dialogue that sounds like a song lyric. I just began this long conversation with the source material. Adrienne Shelley, who is no longer with us, was a huge part of our creative conversations and continued the legacy of this beautiful story and film that she made.

“So I said ‘Yes!’ from a very naive place, and I’m so glad I did.

“Because it has just been truly the reorganizing principle of my life. It’s changed absolutely everything for the better. I’m very grateful this show came into my life — and continues to be in my life.”

A year after the show opened, Bareilles made her Broadway debut as Jenna.  “I remember feeling I wanted to raise my hand to see if I could step into those shoes.

“The beautiful thing about a show is that all of these cast members, band members, crew members, they became my family, man. I was at the theater all the time. It was really exciting and I was very nervous but oh my God! I fell in love with it.”

The result is for all to see, beginning Thursday for a week via Fathom Events.