It’s really exciting starting a new book.

My daughter asked me to write about a girl this time, and I promised I would. I knew the heroine had to be special in some way, and to have a quirky friend, so I looked around at the folk I knew, to see who fitted the bill.

I’m fascinated by people, but I don’t always ‘get’ them, so I’ve chosen protagonists that I want to understand better; it’s my way of exploring what makes them tick. They aren’t modeled on anyone in particular, but they do have the traits of a number of people I know – which might not delight them if they recognise themselves, but I’m hoping they never will.

I always give my characters at least one quirk, like a tag, which makes them distinctive to the reader. Usually it’s a habit, like sniffing or always losing their glasses.

I also try and give each one their own style of speaking. That got a bit troublesome in Black Water, because Dad had a stammer which interfered with the action. In the end I seriously edited him.

The characters took on a life of their own in Black Water, and it was really maddening when they drove the story in a different direction to the one I wanted to go in. They invariably won though, because as soon as I made them do something out of keeping, the story world teetered.

That’s why I find it hard to plot in advance, except in the broadest sense, with lots of room for manoeuvre. I was writing a section of my new book this week and the characters made a whole scene happen without my knowing it was going to. I was blown away by it.

I stopped writing at ten to three, as I always do, and went down to the school gates to collect my son, still in a state of shock. In that sense, I really am a reader who has no idea what’s going to happen next.