I wanted to write a very simple story about a boy, a wolf, a girl, a bear and a forest, so I thought I might set it in the past. I didn't realise that it went back to when I was 10: I used to love the Stone Age when I was a kid and wanted to live in it, and I got rid of my bed and slept on the floor, but I didn't remember it.

The most remote place I've been to was in Greenland. I remember setting out for a solo hike from a small cabin, itself several hours' boat ride from the nearest settlement.

I've wanted to write a ghost story for years, and my main aim was to write the most frightening ghost story that I could think of.

To get the feel of the polar night, I went back to Spitsbergen in winter. I went snowshoeing in the dark and experimented with headlamps and climbed a glacier in driving snow.