LONDON-When Sara Bareilles signed up to write the music and lyrics for the Broadway musical Waitress in 2013, she had no idea it would change her life.

The show - which tells the story of a pregnant, pie-baking waitress - became a smash hit, and saw Bareilles nominated for Tony and Grammy awards.

It also introduced her to her partner, actor Joe Tippett, and legendary musician T Bone Burnett, who produced her latest, Grammy-nominated album.

Now, she’s making her West End debut in the show’s title role, which she’ll play for six weeks from 27 January.

“I really talk about my life in categories - before Waitress and after Waitress,” Bareilles tells the BBC. “I can’t imagine life without it.” Based on Adrienne Shelly’s 2007 film, the musical was a departure for the singer, who’d previously scored hits with autobiographical pop ballads like Love Song and Brave.

“Writing for Waitress was like an exercise in empathy,” she says, “because you would have to find your way into the psychology of a character who felt far away from you.

“Earl, the abusive husband, is a good example: I had to try to understand him so I could give him a song that’s not meaningless.”

The experiment worked: Critics embraced the show’s “joyously life-affirming celebration of love and friendship” and praised Bareilles for writing “one of the best scores in years”.

But writing for the stage and appearing on it are very different challenges, as Bareilles discovered when she first played Jenna Hunterson in the Broadway production three years ago (replacing Jessie Mueller, who originally played the role).

“The first time I played Jenna, I was very attentive to the business of the show - the props, and the technical aspects - because it was all brand new,” she explains.

“So when I got to go back into the show, I felt so grateful to be able to settle into something that felt like deeper storytelling to me.”

Bareilles eventually played Jenna three times on Broadway, with each stint prompting a surge in box office receipts. When she was first announced in the role, the musical sold $1.2 million in tickets in a single day.a