Of all the well-worn tropes of detective fiction, the one that I have the greatest weakness for, both as a reader and a writer, is the locked room mystery.

An actual literal locked room mystery is something of a rarity. Arthur Conan Doyle’s The Speckled Band is one – a girl locks herself into her bedroom at night yet is fatally attacked, with no disturbance to the room at all. Her last, strangled words “the speckled band!” are the only clue to her killer. The Murders in the Rue Morgue is another – an elderly Frenchwoman is killed in a locked room on the fourth floor of her building (and although I don’t want to spoil either story for you, curiously the reveal is similar to the Holmes story). The novel often cited as the best locked room mystery of all time is The Hollow Man by John Dickson Carr, and that also features a literal locked room.

However more common are metaphorical locked rooms – an isolated, self-contained stage from which a small cast of characters cannot escape, limiting the solution of the mystery to a small, fixed pool of suspects. Agatha Christie wrote some of the best known in this category, including two of her finest novels, And Then There Were None, in which twelve people are trapped on an isolated island and slowly picked off, and Murder on the Orient Express, where twelve passengers are trapped in a snow-bound train carriage, and the thirteenth is found dead in the morning.

My own novels betray my weakness for the genre. In a Dark, Dark Wood takes place at an isolated glass house in a remote Northumberland forest. The Woman in Cabin 10 features a literal locked cabin, but also a closed cast of suspects aboard a ferry in the North Sea. And now I can add to that list One by One, in which a group of colleagues travel to a luxurious but remote chalet in the French alps, only to be trapped when an avalanche descends. When members of the group start dying, the twelve (what is it about that number?) cast members are whittled down, one by one…

So what is it that I love so much about locked room mysteries?

Well, as a reader, I think it’s just the sheer satisfaction of trying to solve the “impossible” puzzle. It’s like watching a magician pull off a conjuring trick – you’re shown something that shouldn’t be possible, a dove from thin air, a woman sawn in half, and you have to work out how it was done.

As a writer, of course, it’s fun being on the other end of the puzzle, pitting your wits against the reader to pull off your “impossible” illusion. But there’s also something really satisfying about working with a small cast of characters, and having to use them effectively. There’s no possibility for walk on parts, or convenient extras who can provide the skills or knowledge needed for a particular reveal. You have only the characters you gave yourself at the start of the enterprise, and you have to make sure they have everything you need to get you through to the final revelation. The bonus of course is that you can get to know them really well. Characters who would otherwise have spent just a moment or two on the page can be fleshed out into real people, cropping up again and again. We get to see them change, grow, react under pressure, and come out with surprising revelations. Sometimes, of course, we see them die.

In general my books are quite light on corpses. Frequently the deaths happen off the page, and there’s often only a single murder. One by One is an exception to that, and it has probably the highest body count of any of my novels so far, something you know almost from the very first page, when a newspaper article reveals the number of survivors.

Because that’s the other thing about locked room mysteries. It’s not just a closed cast of suspects. It’s a closed cast of victims too. And when you know from the outset that one or more of that group is going to die, people you have got to know, maybe even got to like, well, that really ups the ante. And that, I think, is the real pleasure behind a locked room mystery – the feeling that a situation has closed like a vice around these characters, with no way out. Until there is.