IN A cafe in Edinburgh's Broughton Street, Camille O'Sullivan is wearing red sparkly brogues and an unnecessary air of apology. "I'm sorry," she says when I tell her I'd seen her show the night before. "It was like a comedy act."

It was entertaining, certainly. It was only the second night of her Edinburgh show at the Pleasance and it was plagued by lighting issues. "I should just have got on with it," she grouses half-seriously.

Actually, I tell her, it just added a leavening humour to the intensity of the show.

O'Sullivan is in town this month to sing songs by Nick Cave. The result is a giddy, dark festival of song and performance (just wait until you hear - and see - her performing Cave’s Stagger Lee), a reinterpretation of the Cave songbook that is by turns impolite and raucous, sad and haunting.

It's taken the Irish singer 15 years to get up the confidence to commit herself to a show just singing Cave's songs. "Because I'm one of the biggest fans too,” she admits. “I don't want to mess it up."

No fears on that score. Even with lighting that doesn't want to play along, she puts on quite a show; a theatrical, noirish take on the Nick Cave songbook that by turns seduces and intimidates.

It's hard to believe that in real life O’Sullivan is "a mouse" as she says, someone who won't even sing at her own parents' parties.

But on stage these days she thinks she now just goes with the flow. "I find as I'm growing older and things are falling apart now, I'm kind of embracing that on stage. Many years ago, when I started, it was about being cool."

That's no longer the case. "I used to think: 'Don't show that fire.' But now I think: ‘Show the bloody fire.’" And so, she does.

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Cave's songs she thinks are the "best theatre ever," full of beauty and darkness. From Into My Arms to The Ship Song, O’Sullivan and her band make them their own.

That’s not always been easy, she admits. And some of the Australian singer-songwriter’s catalogue she always thought was beyond her. Stagger Lee, the most excessive of his Murder Ballads for instance.

"I never thought I'd do Stagger. I love that song, but I thought I will never touch it. I thought people would walk out. And yet eventually O’Sullivan took the plunge.

"Stagger Lee is the nastiest bastard," she says, thinking of its profanity and violence. "I am fascinated that that [violence] is in all of us." Cave himself, she points out, has said that he even finds it a horrific song. He performs it as if it is an acting role and O’Sullivan does too, she says.

“But I really enjoy it now. It's one of the moments I revel in. Before I was almost flagellating myself. 'I shouldn't sing it.' Now, I'm like, 'Oh yes.'"

O'Sullivan is entertaining rambling company. Her thoughts and words circle around and dart off at tangents. That may be because she's sleeping on the floor in her flat this week because she has given one of her band members the bed.

O'Sullivan has been coming to Edinburgh for years and she's now one of the Fringe's most reliable pleasures.

Originally an architecture student, she started singing after a car crash in 1999 that hospitalised her for months. Inspired by seeing the cabaret singer Agnes Bernelle, O’Sullivan started performing Kurt Weil and Jacques Brel songs in cafes around Dublin. She's widened her repertoire in the years since and Cave is now one of her favourites.

She has met him down the years, she admits; meetings that bring out the mouse in her. "Every time I've ever met them, I can't get a word right. Never meet your heroes. It feels like I'm doing a thesis on the fella."

But there’s nothing academic about how she sings his songs. To do them justice, she says, you've got to own them.

"You don't want to be doing an X Factor. It sounds like bullshit to explain it, but it just means you're as present as you can be. I would hate for there to be fakery. It's my biggest fear."

Camille O’Sullivan Sings Cave is on every night at 9.15pm at Pleasance One – Pleasance Courtyard, until August 25