Starring in her third novel, lesbian private eye Saz Martin is fast becoming a cult figure. Meanwhile, her multi-talented creator is off to perform in two shows at the Edinburgh Fringe Festival
Stella Duffy, crime-writing lesbian and not quite professional blonde, is trying to explain how she evolved from a New Zealand actor who also wrote into a south London writer who also acts.

"In NZ, I was a proper actor, I also wrote plays... I just didn't call myself a writer. I came to London, I did stand-up comedy ... that was writing but I didn't think it was. I did impro... like writing but live. Then I wrote a book, which someone bought. When I had my name on the side of a book, then I knew I was a writer." Amazingly, words almost fail her. She shrugs, "I guess it's my post-modernist, Nineties, Renaissance-woman type thing."

An overweight teenager with red hair and freckles, after 11 years of Buddhism, two and a half years of therapy and a bucketful of yoga, she has arrived in her thirties with only the freckles intact. Duffy is the youngest of seven and shares a birthday with Karen Carpenter. Success crept up on her. Her name now on the side of a third book, she tops Cosmopolitan's smartest, sexiest Scribe with Vibes list.

Perhaps it was a mistake to have her photographed during the interview. Out on the street she was charming and rather cute. In front of the camera she is all floating eyes and flashing hair, her shell-pink nails conduct a wild ballet of croissant flakes across the table and her laugh takes on a piratical bass note.

Before Duffy the novelist, came Duffy the performer. "It astounds me when people think that this me - the performance one - is what it is all the time," she says ingenuously. I breath a sigh of relief. Duffy in full swing is delightful but exhausting and liable to shout "masturbate" in the middle of a tea shop, as she has just done.

It is hard to believe that someone who clearly relishes live performances could cope with the loneliness of fiction writing. She disagrees. "I like doing things by myself. There is no one telling you if you are funny or not." The result of this solitude is a clutch of raunchy crime novels centred on inept sleuth Saz Martin. The third, Beneath The Blonde, has just gone on sale, and it is becoming clear that Saz's lack of prowess as a private eye is more than made up for by her activities between the sheets. Yet, the sex has become less graphic since Calendar Girl, the first novel. What has happened?

"My mum reads them," admits Duffy, who minutes earlier had been expounding the importance of total honesty. "I wrote my first book not expecting anyone to buy it, and was really, really open in it. Then I realised everyone was reading it and, because people are stupid, thinking it was me. I don't have a great sex life," she pauses, "well, that's not fair, I have a very good sex life, but I don't have a fiction sex life. I've been in a monogamous relationship for seven years and part of me would love to shag around like that, but if I get to write it, I don't have to do it." So, what did her mum think? "She read Calendar Girl and said she was going to come back as one of those women, because now she knew what she was missing." Duffy collapses into laughter.

The couplings of Calendar Girl may be vivid but they are also mushily romantic. The book's style is oddly reminiscent of early Jilly Cooper; fast plot, breathless sex and acres of puns. The crime, while cleverly constructed, is a side issue. "I didn't think I was writing a crime novel when I wrote Calendar Girl, I thought I was writing a girls' love story," confirms Duffy. "It went horribly wrong and one happened to be dead, so I had to invent a detective and then I had to put her on enterprise allowance so that she could be wrong." She sighs: "Yes, I read Nancy Drew and Trixie Beldrun as a kid, but I wanted to write beautiful, sexy, gorgeous literary work."

She abandoned her literary pretensions long enough to find her feet in crime writing and the subsequent two novels, Wavewalker and Beneath The Blonde are excellent, fast-paced mysteries.

Duffy is mesmerised by all manner of repulsive low-life; her creations range from homicidal self-help gurus to raddled, has-been actresses, but her real scorn is reserved for characters who fail to "come out". "I hate that there are a lot of people not out, I think it's morally wrong." Always one step ahead, she has managed to come out not once but twice. "I'm not only out, but I'm out of the gay community as a dyke who has in the past fucked men and quite liked it. What's the big deal?"

Beneath The Blonde follows a vodka-drinking, coke-guzzling, rock band on their world tour while Saz tracks down a stalker with designs on the luscious lead singer. Duffy has hitherto been portrayed as gorgeous, pouting and, frankly, up for it. Is the novel based on personal experience? "The places are real, but I have sadly never had the money to have that much cocaine," she says ruefully. "If I do have creepy fans, they certainly don't send letters. Maybe they just cut out photos." The thought doesn't seem to bother her much. Would she like real fame? She nods vigorously. Daytime TV? Richard and Judy? "Ooh yes! I love Richard and Judy," Duffy the novelist turns back into Duffy the performer. "But when they introduce me they'd say: 'This morning we have [her voice hushes in mock concern] A LESBIAN... on the show.'"

Saz may be Cosmo readers' private dick of choice, but Duffy has abandoned her for a while to write "a magical realist novel with lots of shagging". She also has two shows at the Edinburgh Fringe: Close To You, about a woman obsessed with Karen Carpenter; and Sub-Post Office Of Death, an improvised comedy.

Meanwhile, Beneath The Blonde is tipped to be the beach-read of the summer. Richard and Judy, we await your call.

Performances of Close To You, 4.30pm, 8-30 August, and Sub-Post Office Of Death, performed by Spontaneous Combustion, 6pm, 8-30 August, will be held at the Assembly Rooms, Edinburgh. Beneath The Blonde is published by Serpents Tail, pounds 8.99.