Charlotte Grimshaw on the family dynamic that inspired her new novel The Black Monk

As I came up from the Bay, I thought about my novel and suddenly conceived it as my own Black Goya. I was grappling with a problem not likely shared by my fellow middle-aged women dogwalkers: “Morning Charlotte! Why the long face?” Well, the thing is, I’ve frightened myself half to death. I’ve written a novel about a character who was befriended, as a young person, by a sinister figure.

It was a fictional idea that began to exert a psychological effect. As soon as I started writing the story, I was uneasy. The Black Monk was experimental in its structure, and I enjoyed the conceit that it was forming itself, in the way fiction really does, by percolating away in the subconscious, taking shape and presenting itself to the conscious mind.

But it was also forming, fictionally, in another way: my character, a children’s fiction writer, was emailing the manuscript to herself each day, and had the idea that someone was hacking in and approaching her in real life to affect its outcome.

The book took on its own life, and it seemed as if there was something reckless in it. It felt as if art could become life.

I described The Black Monk as a “fictional confession”, and I never stopped playing around with that idea. This situation – my uneasiness, my sense I was doing something odd and even reckless – continued as I finished it.

The Black Monk by Charlotte Grimshaw is on sale March 3.

I came up from the estuary with the dog covered in black mud, and I thought, I’ve been in the grip of something strange. The Shadow, for God’s sake? I feel as if I’m ensnared in my own fictional trap.

I remember lying in bed at night as a child, terrified. I was always afraid there would be an intruder. I generated fear by making up stories. Now, I’d frightened myself again. I’d invited the tiger in to tea. I feared some consequence.

Guilt is a prison. I tried to help my brother, but I couldn’t stop him. I didn’t save him. When I visited him for the last time, I begged him to save himself. I am haunted by the last visit, by his suffering and pain. I stayed with him, but then I left him there. I always had to leave him there.

Ever since, I’ve been writing a story that seems, in the oddest way, to invite the reader to see that world of guilt. And I suddenly thought, as I walked out of the estuary, how strange it is to live with this sense of oppression.

I wanted to paint guilt as black as it felt. I’m just a novelist; I have never (not yet anyway) met the Devil. Perhaps it’s true, we all have a Shadow. But this is what I wanted to express: I have met people who have the qualities that motivate the worst wielders of power.

A novel is that strange blend: it’s all invented and it draws on life experienced. Finishing it, I can walk away. The walls are gone, and that gives me joy.

This is the power of fiction: if I can describe the prison bars, I can write the open door.

The Black Monk by Charlotte Grimshaw is out on March 3.