"Do you mean our writing partnership?" I've just asked Nicci Gerrard about the creative bond she has shared with her husband, Sean French, since they embarked upon their debut thriller, The Memory Game, almost two decades ago. But I could just have easily have been asking about their personal relationship. For just as their nom de plume, Nicci French, is a clever composition of both of their names, it's clear their 17 novels to date are very much a product of combined imaginations.
"When we first started writing we didn't really know what we were doing, so we just cobbled together a method of working," Gerrard says. "We don't actually sit down and write together. We plan together very intensively and then when we start writing we sit as far away from each other as we possibly can. We write separately and pass the book between us. That's how we did it with The Memory Game and that's how we've continued to do it because it seems to work, and there's no other way of doing it. We couldn't write sitting down together, so in that sense it's a bit messy but it's a tried and tested method."
Having met when they were both working for the New Statesman in the 1980s, French and Gerrard married in 1990 and tried their hand at fiction a few years later. "We were already writers, as we were journalists, but we also wanted to write books," French says.
"The Memory Game started out as a kind of experiment, as we wanted to see whether it would be possible to write together. After a few books, we realised we could do it and there was something really interesting about it in that we were able to write in a way we couldn't if either of us did it on our own. We could somehow go into different areas."
After previously concentrating on standalone novels, French and Gerrard launched a series in 2011 which centres on troubled psychotherapist Frieda Klein. "Technically for us, it's a different thing," says French. "In the past, with each new book we'd have to invent a whole new world because it's a new character so readers would gradually get to know the main characters. With a continuing scenario, it changes your relationship with readers because they almost develop their own stake in the story and have their own wishes for what's going to happen."
Beginning with Blue Monday and including the new Thursday's Children, each instalment in the planned eight-part saga - with the possible exception of the mysterious final volume - is named after a day of the week. "That's no coincidence," says Gerrard. "It's about time passing and how time marks people and marks characters, how they suffer and how they change, so that, for example, a teenager will be a young woman in her mid-20s by the end."
"It's interesting to see how things change," French says. "If you take a character like Miss Marple or Hercules Poirot, they never change or age and you can read their books in any order. It also means we have more of an interest in it. We made up this world and now it has a kind of mind of its own, which goes off in directions that you don't always expect.
"The characters don't always do what you tell them to do and you realise you've got more to learn about them, so there's that odd feeling where you suddenly feel like you're in this strange world that is slightly out of your control."
Along with the daily theme, each book features one of London's many buried or forgotten rivers such as the Tyburn in Tuesday's Gone and the Wandle in Waiting for Wednesday.
"They're like sources of life that have been blocked off and tarmacked over," says Gerrard. "For us, the city is like a body and the hidden rivers are the blood that flows through it. They're also like secrets and that's very useful in Frieda's world, which is all about secrets and the hidden things we don't even know about ourselves that can nevertheless affect us."
With a dark secret from Frieda's own childhood coming back to haunt her in Thursday's Children, Gerrard and French believe their choice of a psychotherapist as a central protagonist has provided them with a special perspective on the impact of crime.
"We were always very clear we didn't want to have a police officer as the main character," says Gerrard. "That's been done before by so many other authors and we weren't so interested in having an expert solving a mystery, because that wouldn't play to our strengths.
"But on the other hand, we didn't want a Miss Marple stumbling over bodies all the time. So there needed to be a way in which Frieda could be drawn into those kinds of investigations."
With Gerrard and French hard at work on the fifth book in the series, Frieda's adventures have now reached the halfway point. "In Blue Monday the overarching story was set off with Dean Reeve," says Gerrard, referring to the sinister killer who has continued to stalk her ever since. "That will play itself out all the way through to the end, so there's a big, dark story that's going on and then there are all these smaller stories around that."
"We've often wondered what it would be like to be a therapist and what toll it eventually takes on you when you're hearing about people's traumas day after day," adds French.
"Part of the thing, as we move through the week, is the toll it has taken on Frieda. She doesn't just leave the early stories behind, they're all there with her still. That's not to suggest that she becomes more and more flattened, as she's quite resilient, but there are some things we're planning to do that will take her in some different directions."