And, most crucially, Ms. Bareilles had to acclimate to a new way of working.

Collaboration — and trusting outside opinions — has not always come easily to Ms. Bareilles, partly because of some unfortunate experiences with record companies.

“As a writer, I tend to be very protective of my work until it’s completely finished, fleshed out, until I’m ready and willing to go to battle for it,” she said. “That is less helpful in this process, because this show depends on the music serving the book, the book serving the music, the music serving the actors. Everything has its mirror image.”

While she can still feel herself bristle at feedback, she has become less precious about her ideas, especially as she’s become more comfortable with her creative team colleagues, who include the book writer Jessie Nelson. She now understands that they’ll be revisiting everything dozens of times before the show opens.

During a Saturday rehearsal, less than three weeks before the start of previews, one of those changes was being worked out. Since its run last year at the American Repertory Theater in Cambridge, Mass., “Waitress” has been altered considerably. Scenes rewritten, moved and jettisoned, parts recast, rethought and deepened. The biggest difference: a new movement vocabulary from the incoming choreographer Lorin Latarro, which attempts to unlock the fantasy life of Jenna during her pie-making études and to turn a static show into one where the ingredients (the ensemble and band, in particular) literally swirl on stage.

Although the music has changed the least among the creative elements — there’s just one new song — that doesn’t mean Ms. Bareilles’s workload has lightened. Songs have shifted, lyrics have been reworked. And even the smallest staging adjustment can mean a tweak to the music.

After a run-through of “Never Ever Getting Rid of Me,” a first act song given to Ogie, the oddball suitor of one of Jenna’s fellow waitresses, Ms. Paulus called over Ms. Bareilles. The pie-making reverie Jenna then falls into was no longer as romantic, but a little more salty and quirky. The music, namely the underscore, now needed to match the action.

Ms. Bareilles huddled with the piano player and drummer. She suggested a change in the orchestration to give the song a bit more oomph, humming along as they tested it out. A few moments later, after the actors had gone through the scene again, she was asked if the change had done the trick. “We shall see,” she said, knowing full well more alterations were bound to come.