Writers of gruesome crime thrillers, says Frances Fyfield, need to have a sense of humour. This makes sense after a long day thinking up ways of murdering and maiming people in grisly ways. You must need a bit of light relief.

That helps explain why gatherings of crime writers tend to be, in her words, "rather good fun". She has high hopes of her visit to Harrogate Crime Writing Festival this month.

"One of the things that distinguishes them from other writers is they tend to get along. It's not like going to a gathering of romantic novelists where they are all bitching about each other," says the criminal lawyer turned bestselling crime novelist.

She's a full-time writer now, having given up working as a lawyer for the Metropolitan Police and the Crime Prosecution Service some time ago. Until recently, she did one day a week for the CPS, although she's abandoned even that now to divide her time between homes in London and Kent.

She was a late starter as a writer. "I would never have considered it, even in my youth, because it's too bloody unsafe," she says. "I come from a background that had bailiffs at the door occasionally. I always knew I wasn't going to be a person who had debts. Writing and stuff was what other people did."

She read English at Newcastle University at a time when you didn't have to know what you were going to do at the end, she says. You simply told yourself that a job would turn up somewhere.

Fyfield enrolled on a law course, leaving to work in such diverse jobs as shop assistant and theatre dresser, before eventually finishing her studies and emerging with law qualifications.

She answered an advertisement for solicitors to work with the Metropolitan Police, where she was one of the few women working in this area. Her interest in crime was aroused as she progressed from minor cases of indecency and gambling debts to those involving brutal rape and battered wives.

It took the end of a short-lived marriage to spur her into action and get writing seriously. "I suppose writing was always there. When you're a trainee lawyer you find yourself writing your boss's letters," she says. "I wrote bits and pieces for magazines, and then short stories."

Crime writing came easily because of her legal work. "I'd read Simenon and all the English classics but I wasn't a person who was addicted to the crime genre," she says.

Working in crime by day and writing about it by night provided a fascinating contrast. "You'd do the real thing during the day and fictionalise it in the evening, and maybe work towards having a happy ending in your own fiction while you knew there wouldn't be one in real life," she says.

At Harrogate Crime Writing Festival, she'll be taking part in a debate about series characters. She has several to her name - Crown Prosecutor Helen West and solicitor Sarah Fortune.

Neither character is Fyfield, although she admits to some similarities with her fictional creations. Shopping and dislike of domesticity in West's case; a dread of being locked in a situation, physically or mentally, in Fortune's.

Helen West featured in several TV movies, played by Juliet Stevenson and then by Amanda Burton. Fyfield appears not to have been overkeen on the results. "They have a rather shallow life on film because it's necessary, a great simplification of what goes on in a book," she says. "I'm sure it's a good idea if it works and is beautifully done but, in my case, it's not so well done."

What's undeniable is that women have made a killing as crime writers. Fyfield doesn't think they're better than men necessarily. "But historically, they're associated with being the best at it because of the role models like Agatha Christie. Women can see a precedent to follow," she says.

"In the English crime novel, women excel because they know more about fear. Men might express it because they have to. Women are told not to go down that dark alley from the word go. They're supposed to look after the children and told they're just very good if writing the 'footsteps in the dark' type of novel."

All this makes the psychological crime thriller market a very competitive one. Not just sales but reaction from readers can be rewarding. "It's good if someone says they really enjoyed the book and understood what I was talking about," she says.

She's finishing her next book due out in October, and had reached chapter ten when we spoke. "It gets harder and harder. It's a lonely addiciton. I'm trying to get my serial killer to behave," she says.

* Harrogate Crime Writing Festival runs from July 21 to 24. Frances Fyfield joins Mark Billingham, John Sandford and Cath Staincliffe to discuss the pros and cons of the series character with Simon Brett on July 22 at 5pm.

Tickets in person from Harrogate International Centre box office or by phone on (01423) 537230 (Mon-Sat 9am-5.30pm) or via the website www.harrogate-festival.org.uk/crime