The platinum-selling pop singer/songwriter Sara Bareilles is also the Broadway composer Sara Bareilles. And the national tour of Waitress, the Bareilles-scored musical based on the 2007 cult-classic film, puts its wares on the windowsill at the Forrest Theatre from Feb. 13 to 18.

Was it hard to go from crafting pop tunes to making a musical? "It felt initially like a tall order," she said in an interview recently. "I come from a school of songwriting where you write pretty much from your own viewpoint."

In a musical, on the other hand, the songs need to interpret the characters, "get into them as other people," she said.

This is where the interview went from fun to beautiful. "As I kept working," Bareilles said, "I found I could crawl into the psyches of all these delicious misfits." She connected with lead character Jenna — vulnerable, entrepreneurial, loyal — "and she was my way in. Songwriting takes empathy, understanding what a character is, and Jenna taught me how to do that."

"She Used to Be Mine," one of the most heartfelt songs in the show, is sung by Jenna late in Act 2, as she muses on the person she wanted to be and the person she has become.

"That was the first song I wrote for the show," Bareilles said, "and it ended up serving as a portal for Waitress, leading me deeper."

Bareilles credits Adrienne Shelly, who wrote and directed the movie but died before it debuted at the Sundance Film Festival. "All her characters are sympathetic — even Earl, our so-called villain, is a complicated, scared, vulnerable man. He sort of breaks your heart.

"It's nice to see characters who live inside the gray area, nice to have a show that tells you you can love someone who makes mistakes," Bareilles said.

She joins a growing number of pop composers who've taken up the Broadway musical, from Duncan Sheik (Spring Awakening) to Elton John (The Lion King).

"It's exciting to be part of this revitalization of the musical form," she said. "I feel incredibly embraced by the theatrical community who work in our regional theaters and Broadway. It's hard work for not that much money, and they do it with so much love and joy."