By Mark Savage
Entertainment reporter, BBC News
"I didn't realise until I was backstage," says Sara Bareilles from behind her piano at London's Bush Hall, "that I've come dressed as Dorothy from Wizard of Oz".
Bareilles says songwriting is the most sacred thing in her life
Given that she is wearing a T-shirt with the legend "love your body already" printed across the front, the Californian songstress is hardly a facsimile of Judy Garland (although her hair is in pigtails) - but her rise to fame has had a fairytale quality.
Quite by accident, she garnered serious internet buzz thanks to a shaky handheld video taken when she supported British act Aqualung in San Francisco.
"I had never even heard of Sara Bareilles before," said Brad, who posted the clip on YouTube last May.
"So when she walked out alone on to the stage, I wasn't anticipating anything special.
"And then something amazing happened. She opened her mouth to sing... and this incredible voice softly drifted into my ears."
The video shows Bareilles perform her tearjerking ballad Gravity sat alone at an electric piano, as the audience listens, enraptured, in complete silence.
Despite the amateur quality, it soon became the artist's calling card.
Her manager at the Epic record label even used it to promote Bareilles to his colleagues. "I would show it to everybody," he told Billboard magazine last year.
"When you see her live, she makes believers."
Having created a word-of-mouth buzz on several music blogs, Bareilles graduated to the big time when her first single, Love Song, was picked for a US TV ad.
"When the album first came out, it had a little bit of success and it was doing great - in my mind, at least," the singer-songwriter says.
The singer reveals a sarcastic sense of humour in her live show
"Then Love Song was featured in a commercial and since then, everything has just been totally surreal."
The single - a hook-laden slab of FM pop - has now landed in the UK top 10, with critics comparing her easygoing pop to the likes of KT Tunstall, Fiona Apple and Maroon 5.
Her concert in west London is her first headline slot in the UK, and it shows Bareilles to be a confident performer.
At times, the heavily-formatted pop drifts towards comfortable blandness, but a
gutsy, bluesy cover of the Beatles' Oh Darling is the stuff of legend.
But the crowd are enthusiastic - and a sizeable number know enough of Bareilles' lyrics to drown her out on the big choruses.
The response to her music has been "amazing" and "unexpected", she says.
It is all the more gratifying because of the struggle she endured to get the record on to the shop shelves.
On her website, the singer says she walked away from recording sessions with "gnarly battle scars" after "fighting, and seeking compromise" with producer Eric Rosse - famous for his work with Tori Amos.
"It wasn't actual fist fights," she explains, "but there was definitely some actual trauma.
"I chalk it up to growing pains. I thought it was going to be so easy and there were just so many things to learn."
Bareilles admits she was "naive" going into the studio but, strangely, she had written a song anticipating the pressures of working for a major record label five years earlier.
Bottle It Up, which appears on her album, sees a record company executive advising the singer to suppress her artistic impulses because "sensible sells".
Bareilles' album reached number seven on the US charts
"I know that it's your soul but could you bottle it up?" the unnamed music mogul asks.
"I had a lot of fears about being overwhelmed by the industry," Bareilles admits.
"But the chorus of that song came to me right before we started the record, and it has the answer to those fears: 'I do it for love.'"
"I feel very passionate about this so, as long as my intentions were good, I knew everything would happen the way it was supposed to."
In the end, Bareilles did come under record company pressure to write a big hit single after she thought the album was finished.
The track she delivered was Love Song - but the attentive listener will realise the song is a steely kiss-off to her bosses.
"I'm not gonna write you a love song 'cause you tell me it's make or break," she sings, with a wicked glint in her eye.
"I like the juxtaposition of saying something angrily but putting it in a fun, pop melody," the star explains.
"Like finding a chilli in an ice cream? Yes, but hopefully the flavours match!"
Sara Bareilles' album, Little Voice, is out now.