Inside the hidden heart of PJ Harvey

The avant-garde singer-songwriter has teamed up with an old friend to shock again. Nick Hasted reports

Polly Jean Harvey and John Parish are sitting across from each other in a quiet hotel with the comfort of 20 years' acquaintance. Harvey, of course, is the star, the woman whose Mercury-winning album Stories from the City, Stories from the Sea (2000) isn't even a career highlight. Her debut Dry (1992) marked her territory with roaring blues songs such as "Sheela-Na-Gig", referencing Gaelic female church gargoyles with stretched-apart vaginas and mad grins. This wasn't going to be a ride for faint hearts. Harvey's peers over eight cussedly unpredictable albums have been the likes of her friend Thom Yorke, and Björk: musicians who have kept large, loyal audiences as they pilot pop careers into the avant-garde wilds.

This doesn't make her an easy interviewee. Her personal life and lyrics are off-limits. The image that sticks in my mind for days afterwards is of Harvey looking at me with unhappy concentration, turning questions suspiciously like unexploded bombs. Parish, by contrast, is an understated, relaxed Bristolian. He has co-written a fine Eels album, Souljacker (2001), produced or played with Goldfrapp and Giant Sand among others, and made three solo LPs. But his and Harvey's collaboration has been the closest for them both. A Woman A Man Walked By is their second joint album, 12 years after Dance Hall at Louse Point. He also produced To Bring You My Love (1995) and White Chalk (2007). They met after she precociously invited his band, Automatic Dlamini, to play her 18th birthday party. What was the teenage Polly like?

"A very self-possessed, interesting character," he remembers. "She already had good natural voice. I was immediately attracted to that – I wanted to get to know her. We felt comfortable sharing ideas. As soon as there was an opportunity to get her to join my band, I did."

Harvey, unsurprisingly, wasn't feeling self-possessed. "No," she says, casting her mind back. "I felt I knew where I was going creatively. And felt quite driven to go there. I didn't know I was a musician then. I was trying to think of earning a practical living – studying English Literature, possibly going into teaching. But I knew music would be with me throughout my life."

Harvey was raised in a happy, music-rich home in Corscombe, Dorset. But when she moved to London around the time of Dry, tasting urban life, the music business and fame all at once, she broke down. Looking at herself in the mirror, she hated what she saw. There have been other reported collapses since, as with her wrenching mid-Nineties break-up from Nick Cave. Harvey is wise to shut the door on her private life now. In those early days, though, she mentioned how dissatisfaction with her work caused much of her stress. "If I play Rid of Me [1993] at home," she said, "by the end of side one, I can't breathe." Does music still cause her pain?

"I certainly don't feel in pain listening to A Woman A Man Walked By," she says. I go through huge doubts when I'm writing. But I know that's natural now. What has changed with both of us is our confidence and our ability. Listening back to Dance Hall, I was really astonished at how much better we are now."

Parish was the producer for two of Harvey's most crucial creative about-turns: To Bring You My Love, when she split the PJ Harvey band and found a more graceful blues muse, and White Chalk, with its piano songs inspired by Dorset and her grandmother's death. For A Woman A Man Walked By, as with Louse Point, Parish provided the music, leaving lyrics and singing to Harvey. "I write more extravagant music than I would for other singers," he explains. "Polly's expansive range can match it."

A case in point is the segue between "The Chair", on which Harvey's Arabic wail sinks to the lonely voice of a drowned child's mother, and her old woman's quaver on "April". "It never feels as if I'm acting," she considers. "I don't try and inhabit a character that I have a visual picture of. The best way I can explain it is that I want to make this song live and breathe so it's its own entity and doesn't need me. I find the voice which makes it true."

"A Woman A Man Walked By" is a delirious example of this, a manic blues in which Harvey verbally lacerates a "hermaphrodite" with "lily-livered little parts". "It's swaggering, exciting, vibrant, dirty, funny and vicious," says Parish. Never more so than when the pint-sized Harvey hollers at her quivering victim: "I want your fucking ass!" "It's my mum's favourite bit as well," she agrees, in her more usual prim Dorset tones.

Turner-nominated artists the Chapman Brothers have directed the video for another blues tear-up, "Black Hearted Love". But Harvey's main visual muse remains photographer Maria Mochnacz, a student friend in her days at St. Martin's College of Art. They have collaborated on an ever-mutating PJ Harvey persona, as the real woman withdrew from the public gaze: from the perhaps drowned woman in red of To Bring You My Love to the distracted modern office girl of Stories from the Sea... and the demure Edwardian-dressed woman sitting on White Chalk's sleeve. Uh Huh Her (2004) was stuffed with self-portraits of Harvey, camera always in hand: Warholian in her self-chronicling, Bowiesque in her attention to image. On stage too, she's a glamorous world away from the carefully controlled Polly talking to me. With her personal life hidden, it leaves her records as mysterious things, with words and pictures, howls and dresses to decode: like paintings, more than autobiography.

"Well it's lovely to hear you say that, because that's exactly the way I feel a lot of the time," she says, pleased. "I under-write, to leave space for the listener to fill in the picture. And I think of the whole presentation of the music – how we look on stage, everything. It's not crucial, but it's not a joke. It enhances the songs, and gives people a lot of pleasure – me included."

The new album's "The Soldier" meanwhile breaks new ground for Harvey, suggesting political engagement. Though set in Korea, it resonates with current conflicts. "I've always been very concerned with what is happening in the world," she claims. "But it's only at this age – I'm nearly 40 now [she's 39] – that I feel qualified to open my mouth. Before I wasn't well-informed enough. I felt... not wise enough."

"Leaving California" draws on this West Country artist's temporary 2005 relocation to the West Coast. Surprisingly, Harvey loved LA. "I really enjoyed it. There was only one other person that I ever walked past as the Californians drove, and it was Tricky! He lived a block down. I'd be walking to the supermarket, and he'd be coming back on the other side of the road, going, 'Awlright!'" I'm fascinated by moving to new places – the way painters move somewhere for different light. Then when I go back to Dorset, it's like seeing everything for the first time. I certainly feel rooted in Dorset, and nature's cycle there. Unlike cities, you notice the change of seasons overnight. From bird-song."

Something Bob Dylan said in The Independent last week seems to apply to how Harvey likes to see her relation to her music. Nobody looked for life-changing depths in his songs any more, he suggested: "Images are taken at face value, and it... freed me up." Similarly when Harvey sings "I volunteer my soul for murder" on "Black Hearted Love", you no longer believe a soul's at stake.

"Actually, I read a wonderful quote from Dylan," Harvey says. "He said that whether it was marrying your half-sister or self-punishment, he felt able to feel what that would feel like, and write about it. I feel I can inhabit all manner of situations and write about them too, whether I've experienced them or not. You don't have to be suffering in order to orchestrate suffering [in art]. But one can feel it's important to express suffering – to reflect what it's like to be human on planet Earth."

A decade back, recovering from the shock of her split from Nick Cave, Harvey told a journalist she'd put "more of myself" in her songs than ever before, and planned to go deeper still. She seems to exist more in the light than that darkness, these days. Her ambitions are more modest. "It's a long journey, one's life, especially a writing life, and you explore all the different avenues of trying to reach some source of strength. I'm not a purely autobiographical writer. But I like to feel I can give voice to an emotion when maybe someone else can't, and then it's a nice thing for them to listen to. I'm very happy just to do that."

'A Woman A Man Walked By' is out on Monday on Island Records.