Many years ago, when I was at university, I wandered into a room where a friend was looking at a picture. I don’t remember if it was a photograph, or an illustration in a book; it’s a sufficiently long time ago that it probably wasn’t on a screen.

Anyway, I looked over his shoulder at the picture: a fancy, black, metal gate. Across the gateway’s arch, large letters spelled out “Arbeit Macht Frei”. I looked at the picture, and I laughed.

(You may already be running ahead in this story; stick with me.)

My friend asked why I was laughing.

Well, I explained. I’d always thought that sort of attitude - the mindset that built gateways for The Poor Working Class and put Improving Moral Epithets over them - was peculiar to Victorian Britain. To the nineteenth-century mill- and mine-owners who, simultaneously exploitative and paternalistic, squeezed workers dry while espousing the merits of hard work, and self-improvement. The practice of putting these slogans on doorways and arches was something I’d always found grimly amusing.

And - now - apparently it wasn’t just Britain. Even in Germany, there had been equivalent factory-owners who probably disregarded safety and paid their staff a pittance even as the gateway promised that Work Sets You Free. I imagined the owners waffling into their handlebar moustaches and congratulating themselves on the excellent opportunities they afforded the local labourers. See, if you just work a little harder for a little less money, your opportunities will be endless! If you die poor, it is because you didn’t work hard enough!

My friend - whose Jewish father had fled Europe in the late 1930s, and whose aunts and cousins had not and had died in concentration camps - said to me: you do know that this is the gateway to Auschwitz?

No, I had not known.

“Arbeit Macht Frei” wasn’t another piece of Victorian-era hypocrisy. It wasn’t the pompous moralising I’d assumed. It was a sick joke, a lie to give hope to the doomed people for whom no amount of hard work would ever, ever make a difference.

I don’t remember what I said, or how I reacted. I do remember how I felt: a sudden, sickening drop as everything changed. But, of course, nothing had changed; nothing except my own viewpoint.

I don’t imagine I will ever write a story set in a concentration camp; I don’t know enough, and I don’t feel the stories are mine to tell. But that moment, that drop, is something I’ve been looking for in horror fiction ever since. The pinpoint sentence when a single piece of information causes everything to become different.

When the shadows resolve into a shape. When you find the killer’s plans and realise they are in your own handwriting. When you realise the calls are coming from inside the house.

It might be a twist ending, a single set-piece in which the entire world comes crashing down. It might be a series of tiny reveals throughout the story, a building, unsettling feeling of uncertainty. Both play into the big fear: that thing you know? that you’re certain of? it’s not true. The world is not how you think it is.

Those instants of realisation will always, to me, be the essence of good horror writing.