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Sara Bareilles is no stranger to the stage, but she’s trying something new with “Waitress,” the new musical at the American Repertory Theater: writing for someone else. Bareilles wrote all the music for the show, which is based on the 2007 movie written and directed by Adrienne Shelly and starring Keri Russell and Nathan Fillion. The movie tells thestory of a woman who bakes pies as she goes through some personal turmoil involving a deadbeat husband and a very handsome doctor. The new musical is in good hands all around, with A.R.T. maestro Diane Paulus directing, and Tony winner Jessie Mueller in the lead role. Though it was Bareilles’ first time working on a project like this, she says it was a completely positive experience.

How did you end up getting involved?

I had lunch with Diane Paulus, the director, and we talked about the film, which I had not yet seen at the time, and she asked me if I had any interest in writing for the theater and I loved that kind of storytelling. So I agreed that I would watch the movie and see if there was any kind of connection there. After I watched the movie, it felt like it was sort of begging to be musicalized and I fell in love with Jenna’s character and her melancholy and the tone and the perspective of the film and Adrienne Shelly’s work and we began down the road and it’s been a long and crazy winding road, but so exciting and so playful in so many ways. Really challenging and really frustrating, all of the things that the good things in life are.

How has the collaboration process been?

This whole project exists based on a deep dedication to collaboration.… I had a song that I wrote for one of the earlier drafts of the show that was for our wise, older character, Joe, who owns a diner and he’s sort of Jenna’s guardian angel throughout the show, a very salty guardian angel I should say. And I wrote a song for him and the book writer Jessie Nelson so beautifully said to me one day, ‘I think the beauty of the show is we get to have something only you can give,’ and it was the nicest way for her to say ‘you can do better than that song.’ It was so kind and so gentle and so true.

That is such a nice way to tell you to do something better.

I mean, I’ve been an a—hole, too, sometimes. It has not been all flowers and roses. But I really believe everyone’s intentions are in the right place which is good.

Was it difficult to write for other people’s voices instead of your own?

I’ve screwed up left right and center in that. I’ve written duets for the show and thank god that we have such talented cast members because I learned that men’s voices are lower than women’s voices. [laughs] They’re singing up in the rafters and they’re doing a great job, but I have learned. I’ve duly noted for future work.

Was there any part of going through the rehearsal process when you wished you were the one on stage?

Oh, here and there. I think that just comes with working on something you love. I have to say watching Jessie Mueller in this role is just riveting. I’m just so moved by the way she works and watching her do what she does, it’s like being in a master class everyday. She’s astounding in her presence, in her drive, in her patience, her resilience. She’s so consistent. And just really smart and humble. I like her a lot. If you couldn’t tell.

Have you become an expert on pies?

I still can’t make one, but I can eat one really well.

“Waitress” runs this Sunday through Sept. 27. Tickets start at $45. For details, head to americanrepertorytheater.org.