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. She talked to the BBC about how the show impacted her – introducing 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. I can’t imagine life without it. Writing for Waitress was like an exercise in empathy,, 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.”

But writing for the stage and appearing on it are very different challenges, as she 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. 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.”

Sara 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.

For her UK run, she will be reunited with Tony and Olivier Award-winner Gavin Creel (The Book Of Mormon, Hello Dolly!), who plays Jenna’s gynaecologist and love interest Dr Pomatter. However, she said they won’t simply be recreating their Broadway performances.

“The environment has changed, the country has changed, I imagine it will feel different. I’ve had another year’s worth of life experience between the last time I did the show and now, so I imagine those things will infuse themselves into the work in a really beautiful way.”

In the interim, the singer-songwriter has released a Grammy-nominated album, “Amidst The Chaos,” and worked with Star Wars director JJ Abrams on a new TV show, “Little Voice,” which will debut on Apple TV this summer.

“Our first season is done, we’re in mixing and post-production now. It’s about a young songwriter in New York City – I write all the music for her – but it was really an homage to that early stage when you’re just beginning as an artist, playing for crowds for the first time, and being rejected by record labels… all of the juicy bits!”

Sara has also released an EP of songs that were cut or discarded during the writing process for Waitress – including ‘Knocked Up You’, whose lyrics list the side-effects of pregnancy, from acid reflux and backache, to haemorrhoids and “surprise incontinence”. She described the mini-album as a “little gift to fans” who were mourning the show’s closure on Broadway (the final performance was on 5 January).

“We have had the most amazing fan community for this show. They’re ravenous in the most incredible, beautiful way. It was a long road to get to where we ended up, so we wanted to share some of that music.”

And could the musical of the movie go full circle and become a movie again?

“I would love to see that happen, absolutely. And I’m already working with JJ Abrams, so I’ve got somebody to call!”