Sophie Hannah: 'I want you to question my motives...'

The crime writer Sophie Hannah likes to dream up unique and provocative reasons to commit murder

Sophie Hannah knows what she wants when it comes to murder.

"In a straight crime novel, the motivation might be something like 'he wanted her money' or 'he was being blackmailed'," she says, sitting in her warmly lit lounge – a cosy setting incongruous with the subject at hand. "But those are all-purpose, ordinary motives that might apply to lots of people. For me, a big part of writing psychological thrillers is choosing crimes committed for motives which would only apply to a particular person in a particular situation; a unique, one-off motive that is born out of someone's particular range of psychological afflictions. I want my books to explore motives which make people think, 'Wow! Imagine the psychological state you'd have to be in for that to be your motive!' Whereas things like blackmail, jealousy" – she dismisses them with a flick of her hand – "they're rational reasons for committing murder."

In her critically acclaimed thrillers – her seventh, Kind of Cruel, is published this week – Hannah takes pleasure in plumbing the dark, jarring side of human nature, and with her goodies and baddies alike. DC Simon Waterhouse, the genius detective who inevitably pieces Hannah's creepy puzzles together, is socially dysfunctional; his wife, Detective Sergeant Charlie Zailer, is equally clever but has self-destructive tendencies that undermine her career. "My characters all have issues," Hannah agrees, "but I don't see that as weird or abnormal because I think in real life there are very few bland, normal people. I feel I am writing about ordinary people in the real world – but with the proviso that nobody is really that ordinary."

In Zailer and Waterhouse, Hannah has created two fiercely independent-minded, obsessive, deeply and awkwardly in love detectives with ever-active imaginations. No amount of paperwork or bashings by their boss can weaken their resolve to solve the crime at hand, but they often succumb to their own neuroses in the process. ("They're dysfunctional and screwed up," says Hannah. "And the fact that they know how flawed they are hampers them still further.") Their sidekicks are sketched with palpable affection, from DCs Sellers and Gibbs to Inspector Proust, who flexes his sarcasm like a muscle and clutches his "World's Greatest Grandad" mug like a comfort blanket – except for when he throws it at Waterhouse's head.

Hannah's killers are usually driven to murderous action by some aspect of human nature gone awry – heartbreak, loneliness, obsession – but she has outdone herself with Kind of Cruel: when Amber Hewerdine begins a course of hypnotherapy in a bid to cure insomnia, she's soon linked to a murder investigation. Then Hannah throws in another tantaliser: an incident when, during a family Christmas, four people disappear for a day and return without explanation. The combination of the mysteries and the hypnotherapy, says Hannah, "really appealed – it felt almost dreamlike, and kind of sinister in a new way that I hadn't explored before." Her trademark precision-layered structure creates a multi-dimensional maze that holds at its centre a revelation which is truly hair-raising, even by Hannah's standards.

Forty-year-old Hannah, who came to crime by way of Enid Blyton's Secret Seven ("I remember thinking, 'This is brilliant, stories that have mysteries in them are just so much better than ones that don't!'"), Agatha Christie and Ruth Rendell, was initially a successful poet, having published her debut collection in 1995. "I could see that my poetry was better than my fiction at that point," she says, exposing a pragmatic streak that hasn't hampered her creativity in the slightest, "so I wrote poetry and pretty much nothing else for a while. Then, when I had an idea for what became [the first Zailer-Waterhouse crime novel] Little Face, I knew it couldn't be done as a poem." She had written two crime novels 10 years earlier – unpublished, but they got her an agent – so she had another go. "I saw that something had changed. Even after 100 pages, it was in a different league to the rubbish ones I'd written before."

Hannah brings the same acerbic wit, keen observations and wordplay to her novels that she brings to her poetry. In one exemplary scene in which Waterhouse interrogates the killer, Hannah, with a single sentence, makes their raw vulnerability chillingly real: "He had the uncomfortable sensation that he was talking less to a person than to a survival instinct with a human face."

Hannah borrows from life. Sergeant Zailer, she confesses, "is as I would be if I were in her situation, and if I allowed myself to be more ... spiky." She smiles conspiratorially: "I mean, I think all kinds of hideous things in my head, but in a social situation I'm a smoother-over." Waterhouse, meanwhile, is a combination of Hannah's husband, an ex-boyfriend and a policeman she once knew.

And what drives her fascination with the dark side of human behaviour? "I always notice the dysfunctional dynamic of human relationships, because most places where you encounter it, people are trying to pretend it isn't happening. That was just the frequency I became attuned to – I noticed the way people pretend things are okay when they're not, and how people cover for each other rather than admit that they're in a relationship that is kind of dodgy. I find it interesting, the lies people tell themselves."

Already deep into the next Zailer-Waterhouse instalment, Hannah is happily anticipating the detectives' return to ITV next month, in the second series of Case Sensitive. "The actors, Darren Boyd and Olivia Williams, made that relationship come alive," she says. "The awkwardness and the clashes, but also the fact that they really care about each other. They got it just right." As does Hannah.

Kind of Cruel, By Sophie Hannah (Hodder £12.99)

"It's unique: a banned story, one they have tacitly agreed never to mention to one another. It is certainly the most intriguing story the family owns – a mystery that seems unlikely ever to be solved. No progress has been made towards solving it in seven years, and the reasons why this is the case are almost as interesting as the mystery itself......"