A right pair of shockers: Is there a dark side to Sean French and Nicci Gerrard's domesticity?

This is how it begins. You find yourself hopelessly lost, deep in rural Suffolk. There's no mobile phone signal, nor anyone about to ask for directions. Your mind drifts to thoughts of the Suffolk Strangler, of a disturbed reclusive harbouring ill intent, a psychotic young man waiting for ...

Somehow, you arrive at a large, old rectory at the end of a gravel track, on the edge of a small village. As the door opens, out bounds a Labrador. Welcoming you in is a tall, thin man – his dark hair receding, his voice soft and calm. In the cavernous kitchen, warmed by a dark red, four-oven Aga, appears a particularly petite woman with the demeanour of an excitable puppy.

Both could not be more friendly, offering coffee, and perfectly ripe cheese and water biscuits. Then, sitting at the worn, antique pine kitchen table, your fears slowly return. You look outside and notice that much of the back garden has been dug up. Another glance around the kitchen and your eyes scan the heavy copper pots, the stray rolling pin, a sinister rack of carving knives.

Yet on the couple talk, about their escape from London, their four nearly grown-up children, their glittering crime-writing persona, Nicci French. For a moment you think, this is it. This is success, fulfilment, happiness, a marriage and a working partnership. However, something's not quite right. It's all too good to be true. You are, in fact, stuck in a Nicci French psychological thriller, and you are never going to get out alive. I have recently immersed myself in her/their backlist, so it's not my fault, it's their fault – for playing on my insecurities and turning contentment into torture. For making me see darkness where there should be light.

A Nicci French novel – What to Do When Someone Dies is the 11th – invariably makes the worst out of a good situation. Ordinary people's lives are shattered by a blast from the past, or a child going missing, or an over-attentive new lover. Or, as in the latest, the revelation that your husband has just been killed in a car accident, and that dying beside him was, seemingly, his mistress. Except, you, Ellie, the heroine and first- person narrator, can't believe Greg was unfaithful, even if the woman was a prolific adulteress. Slowly you begin to believe he was murdered, but why and by whom?

Aside from murder, a Nicci French novel also comes with plenty of soul-searching and heart-rending emotion – aided, or at least manipulated by the fact that the narrator is always a youngish, particularly articulate woman. It is this combination of domestic crime and fraught and occasionally weepy reaction that has made Nicci French synonymous with the term psychological thriller. But who exactly is, or rather are Nicci French?

Sitting opposite me are the former literary journalists Nicci Gerrard and Sean French – 50 and 49 respectively. Both studied English at Oxford; both then immersed themselves in London literary life, and after Nicci's first marriage failed, leaving her with two young children, they got it together and she had two more children. A move to rural Suffolk followed, as did their first co-written thriller, The Memory Game. About a woman recovering her lost memory following the discovery of a skeleton, it was an instant hit.

Its genesis, according to Gerrard and French, was surprisingly mutual, stemming from a conversation while on a walk. "I think if it had just been Nicci's idea, then she would have written it," says French. "We really did just have this idea together. Because we both worked in the same area of journalism, we'd always done the same thing – read the same thing, passing things back and forwards. We'd read a long article about the whole recovered memory controversy and we were talking about that."

"Before that," says Gerrard, "we had talked about what makes a voice behind a book, and whether or not two people could make a voice. We had this vague idea that one day in the future we'd see whether we could write a book with one voice." "And we thought," says French, "that the idea of recovered memory would be a good subject for a psychological thriller."

"It does seem a bit of a mad idea to say 'we'll write it together,' because people don't do that," says Gerrard. "But it didn't feel that bizarre when we were doing it. It was more like an experiment to see whether we could."

While French had already written two "literary" novels, The Imaginary Monkey and The Dreamer of Dreams, Gerrard hadn't previously done any fiction. However, French's then agent, the late, legendary Pat Kavanagh, recognised their potential and a series developed. Gerrard and French insist that there was no plan or formula, but that the project was always much more organic and messy – just like their lives in the old rectory, with tons of noisy kids. "It really was very unconscious," says Gerrard. "The book we wrote first in some way defined what we were going to do next. When we wrote it we didn't know it was by Nicci French – that there was going to be the name of a woman on it."

"The psychological thriller seemed to be the kind of area of writing that appealed to what we are interested in, what we obsess about," says French. "Interior things, rather than some fantastically clever serial killer, or terrorist group. In a way it felt very natural to us – and then we found that that was the kind of genre we were writing."

The French-Gerrard husband-and-wife crimewriting partnership is not unique. There are the Americans Faye and Jonathan Kellerman, and the Swedes Maj Sjöwall and Per Wahlöö, who were credited with reinventing the police procedural back in the 1960s. Having jointly researched the material, Sjöwall and Wahlöö would pen alternate chapters, often at the same time. But the division of labour in the Gerrard-French household is something of a mystery.

"We never decide in advance who's going to write the first chapter, until the day arrives," says Gerrard. "It's totally random. It can be either of us."

"We don't have some brilliant method," says French, "even at an early stage when we are just talking about it. It's just a very messy process."

Nicci has a study at the top of the house, and Sean works in a shed in the garden. It seems there is a flow of copy between each other's laptops, which is amended and edited until in the end both feel they own it equally, or rather don't own it equally, because it's become the work of Nicci French.

While French admits to finding it a great liberation to write under a different name and in a different genre, Gerrard wonders whether she would ever have attempted to write fiction on her own. Now, though, she has written three solo novels, with a fourth due out early next year. French is currently working on his fourth. As individual novelists, though, sales are not in the same league as Nicci French. Broadly speaking, French writes sophisticated, almost surreal satires on contemporary society, while Gerrard writes personal dramas. In other words, French is addressing current issues, and Gerrard current feelings. Only together, it seems, do they add the deathly violence.

"We'll never say one of us does something and the other doesn't," says French. "Partly that's the kind of contract of us writing together, and secondly it's misleading, because it doesn't really work like that. Everything is shared."

Gerrard offers, "A lot of people think we discover secrets about each other. I don't think we discover secrets. I think there is something about reading and joining in with someone's imagination. You're part of it. It's really disconcerting and exposing, like going into a tunnel together. We agree to set off down this path, then in this kind of strange mental space together, we're groping around."

"We prod each other," says French. "I've been surprised by Nicci's imagination, her willingness to go on and into certain areas."

It's been an oddly unnerving experience, hearing about this creative companionship. Like all pacts, there's a sense you are being told only so much. Which is where I leave it, and head home in the dark.

The extract

What to Do When Someone Dies, By Nicci French (Michael Joseph £12.99)

"...Like death, affairs happen to other people, not me and Greg. Milena Livingstone. How old was she? What did she look like? All I knew about her was that she had a husband who had identified her body at the same morgue as Greg was in. Perhaps she'd been lying in the drawer above him. In death as in life. I shivered violently, feeling nauseous, then went upstairs to my laptop... then Googled her name."