For crime-writing duo Nicci French, the scariest scenarios are when everyday life goes pear-shaped

Lots of us walk around with everyday, ordinary fears - of flying, heights, poisonous snakes and so on. But more often than not, real nightmares consist of mundane situations made surreal.

You brush your teeth, and they pop out. You try to get your suitcase packed because your flight is leaving, but you can't find anything you need and somehow can't get to the front door. You've left something behind, somewhere, and can't work out how to backtrack and retrieve it.

In fiction, these clammy situations make for the most effective form of thriller, and all the better if the main character is otherwise functional and competent.

And when it comes to crime thrillers by Nicci French, the more together and organised a character, the more vulnerable she is to trouble.

I say "she" because protagonists in Nicci French novels are almost always women. While not intentional, it's no coincidence.

When British married couple Nicci Gerrard and Sean French set out to write a crime thriller together back in 1997, it was a fun experiment, a chance to see what they could come up with.

Both authors in their own right, they wanted to see whether they could create a third narrative style. And they hadn't even decided on a name for this made-up author.

As it happened, that novel, The Memory Game, centred on a female character - a middle-aged woman trying, with the help of a psychiatrist, to unravel traumatic memories after the remains of a childhood friend are discovered decades after she goes missing.

"The thing that really inspired us to write together, that we came across together, was this idea of people going into therapy and recovery, and the question around this issue is, how much can you trust those memories?" says Sean.

"Could they be made up, in a way? And overwhelmingly, that happened to women. And so when we were going to write the story we had to have a woman as the main character in that book."

So it felt right that as a single author entity, the name was a feminine one - her first name and his surname.

"We sometimes wonder, had our first idea been about something that had to happen to a man, would we be sitting here talking to someone about these books by Sean Gerrard that were completely different?" he says.

Sitting side by side, the pair finish each other's sentences, and agree on almost everything. It's not surprising that they're now on a publicity tour for their 27th novel together. Or perhaps it's more akin to a miracle. They plan and research together, but go away and write parts of the novel separately.

The resulting books are a deeply satisfying blend of plot and homey detail - characters wear particular clothes, cook elaborate food. There's a gratifying amount of detail around the houses they live in and what's in the fridge. There are, in other words, seams of texture running through what could otherwise be fairly workaday plots.

But they both maintain that it's now almost impossible to tease out who has done what when it comes to the finished product.

"When people read us, it really makes us pleased if they never realised that we were two people, or certainly they can't tell that we're two people," Nicci says.

"Because when we're writing together, we're writing as Nicci French, this strange third person that we've made up between us. And it's not half of Sean and half of Nicci kind of mashed in together with the edges smoothed away - we really have very different voices when we write separately."

Is it, then, a kind of romance - a way of speaking to each other through words on the page?

"This is our way of exploring the world together. But sometimes we're just squabbling like anybody else," Nicci says.

"It's not sweet and cute, you know, we struggle with each other, and we quarrel with each other, and some of our novels come out of those arguments."

In their latest novel, The Favour, protagonist Jude reconnects with an old flame, and suddenly finds herself agreeing to do him a favour, without asking too many questions. Like many Nicci French characters, she's a highly competent professional, soon to be married, the type of person whose path has long been set for her, a path she has stuck to. But once she agrees to do what Liam is asking of her, things unravel quickly.

"One of the things that we try and do in many of our novels, is we get a character who has often been quite in control and happy with their lives - very dangerous in a Nicci French novel - who feels that they know where they're going and things are sorted out, and they feel quite safe," Nicci says.

"And we both feel nobody is that safe. Life is very fragile. Nobody is that normal, life is very strange and we take characters and we crack open their life. And in so doing, they kind of crack open themselves. And then they can discover hidden parts of themselves they never knew existed, which is quite scary, but also quite exhilarating, I think, that you can't get to the end of yourself."

This, she says, is the kind of book that has always appealed to them both, much as they love a good police procedural. Nicci French books don't focus as much on cops and crime scenes as on the everyday people dealing with the fallout. An ordinary wife and mother who finds herself erasing her presence from an apartment - scrubbing surfaces and wiping doorknobs. A woman unknowingly suffering from bipolar disorder. The young doctor in The Favour who drives to a remote cottage, and then realises she has no idea where she is or why.

"A lot of that is about a kind of intimate dread. It's about taking things everywhere, it's not about five days to find a bomb," Nicci says.

"It's not about a threat out there. It's almost like the threat that comes from within, that comes from a kind of ordinary life. What it's like to feel jealous, what it's like to be fearful, what it's like to feel lonely, and then turning the dial on that, they're emotions that everyone can understand, that often do arrive from very mundane conversations that we're having."

And they both agree that this kind of synergy only happens when they take on this third persona, and start trying to place themselves - as Nicci French - within a new plot.

"The thing about what what people wear, what they eat, what you find when you open a fridge door, look under someone's bed, what it feels like to be in that particular street in that particular community - that is really important to us," she says.

"And when we're planning a book, we plan the plot, and the voice of the central character. But we absolutely have to think, where does it have to be set? What does it feel like to be in that room in that house in that street? Even if we don't put in what's inside the fridge, we have to feel that we know that kind of detail."

By now, the process has become almost organic, but it can only happen when they're both in that third zone. Sean says it's like putting on a costume.

"I still find it very mysterious that when I'm writing the content I'm somehow freed," he says.

"It's like putting on a mask and going to a carnival, to behave in a way you can't in your normal life - I feel I can write in a way that's different."

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