Mueller was among the Broadway actors who took part in the A.R.T.'s recent New York City workshops of Waitress. It was previously reported that the Bareilles musical would be her next project post Beautiful.

She played the central role of Jenna, "an inventive waitress trapped in a small-town diner and a loveless marriage. Pregnant and lonely, Jenna finds relief in making creatively-titled pies — and in a budding romance with an unlikely newcomer."

"The workshop was so exciting," Mueller previously told Playbill.com. "There was just this energy in the room. Coming from Diane Paulus and from Sara, there was like an energy — there was a crackling that was happening. I think people are going to be surprised. They're going to get introduced to these characters and be surprised by how much there's a little bit of themselves in everyone that appears in the play. It's just a really hearty, meaty story about life and choices and all the stuff we have to navigate as we go through life. And then, on top of it, there's just this gorgeous music that she's written that sounds like her, but it's so of the world that they've created for the piece.

"Whether or not the world knows it yet — she's already a superstar — but I think that [Sara Bareilles is] one of those people who we're going to look back on in 30 years, and she's going to be in the Rock and Roll Hall of Fame. I think she's that kind of writer, and she's that kind of talent, and it's really inspiring to be around somebody like that. It's inspiring to be around women like that in a business that I think is very difficult. I mean, I'm not in the music business, but I'm in a part of the entertainment business, and I just have so much respect for people who can keep their core, keep their individuality, and navigate a career in the way that they dictate because I think that's what I get from those two women, [Carole King and Sara Bareilles]."

Click here to read more from Mueller's Beautiful exit interview with Playbill.com.  The new Bareilles-scored musical will make its world premiere at the American Repertory Theater in August 2015. As previously reported, it may be headed to Broadway following its Cambridge engagement, although a Broadway timeline or theatre have not yet been announced. 

Finding Neverland director Diane Paulus and choreographer Mia Michaels will reunite for the project. Additional casting and creative team members have not yet been announced.

The New York City workshop cast also included Christopher Fitzgerald, Amber Iman, Bryce Pinkham, Andy Karl, Keala Settle, Dierdre Friel and Barrett Wilbert Weed, among others.

Playbill.com broke the news of Waitress in June 2013 when Tony Award-winning producers Barry and Fran Weissler revealed that they were collaborating with their Tony-winning Pippin director Paulus on the new musical to be scored by Grammy Award nominee Bareilles. The Weissler's Tony-winning revival of Pippin premiered at A.R.T. prior to its acclaimed Broadway arrival.

Waitress will begin previews in Cambridge, MA, Aug. 1. Opening night is yet to be announced. Paulus is the artistic director of A.R.T.

"Corrina Corrina" screenwriter, director and producer Jessie Nelson has penned the book for Waitress.

Visit americanrepertorytheater.org.

Below, Bareilles performs "She Used to Be Mine" in concert and describes how the song fits into Waitress.