At 12 and seven my sons were pyromaniacs, puzzle solvers, pond-life specialists and keepers of small, doomed pets. One day in 2002, at the kitchen table that doubled as my writing desk, a third boy appeared. Nine-year-old Louis Drax spoke just like my sons. He shared their interest in sea monsters, poison and gothic violence. And he was in a coma. He said: “I’m not most kids. I’m Louis Drax. Stuff happens to me that shouldn’t happen, like going on a picnic where you drown.”

Over the next two years, with Eccentric Coma Kid virtually dictating, I wrote a psychological thriller about a child with warring parents who lies in hospital after a mysterious, near-fatal accident. While the police hunt for his missing father, his neurologist struggles to bring him back to consciousness – and falls for his mother.

There are many ways in which to describe sudden, unexpected disaster, but there are few ways of articulating what sudden, unexpected joy feels like. When my agent delivered the news, pre-publication, that Anthony Minghella wanted to adapt and direct a film version of The Ninth Life of Louis Drax, I ordered a gigantic trampoline.

My sons had been bouncing on it for four years when I received another unexpected call. This time it was the very worst kind of shock. The news of Anthony’s death in March 2008 punched a gaping hole, and left the film industry reeling. As the years went by, the project changed hands many times. Apart from “take the money and run”, the best advice – from the start – had been “just keep writing”. The boys grew up, left home, and became the scientists they were born to be. But the film project remained, like Louis Drax himself, in a deep coma.

Then, in 2014, my agent learned that Anthony’s actor son Max – just a boy when I was writing the novel – had been quietly keeping it alive. Most novelists don’t revisit their work: I’m one of them. Plus I have a bad memory. So when Max sent me his screenplay, I recognised the spirit, characters and atmosphere – and encountered a narrative surprise on every page. He had enhanced the Hitchcockian elements of the story, deepened the emotional charge, and planted a scorpion sting in its tail.

Fast-forward a few months: my family and I are in Vancouver, on location at a former psychiatric hospital. It feels apt that the French director Alexandre Aja, best known for his startling horror movies, is filming a twisted mystery in such an echo chamber of psychic pain. Somewhere within a labyrinth of rooms with barred windows and flaking, bubbled plaster, we meet Aiden Longworth, the child actor playing Louis.

“So. As well as a writer are you a beekeeper?” he asks. I tell him I support bees, but sadly not at that level. He seems disappointed but is swiftly over it. “So I bet you don’t know that hummingbirds can’t walk.”

My son Matti and I exchange a glance. This kid isn’t just channelling Louis Drax: he is Louis Drax. Soon the two of them are energetically discussing robotics, plasma, origami and the temperature of the sun. Aiden tells us that later today he’ll be having an MRI scan, then lying in bed in his pyjamas.

“I’ve been doing that a lot. Sometimes it’s the real me but sometimes it’s that guy.” He points to a silicone dummy of himself propped against a wall. It’s disconcertingly lifelike. “They took a cast of my face. It creeps me out.”

“I know how you feel,” says Matti. “My mum stole my voice, and my brother’s, for Louis Drax. I know all about creepy.”

It’s a hall-of-mirrors moment: a young man looking back on the boy self who partly inspired a fictional boy, interpreted by a real boy contemplating his own simulacrum. The nine-year-old and the 25-year-old versions of Louis Drax high-five.

And now, 14 years after coma-boy first appeared at my kitchen table, there’s a film starring Longworth, The Fall’s Jamie Dornan, David Cronenberg’s muse Sarah Gadon, and Aaron Paul from Breaking Bad.

What writer doesn’t fantasise about swallowing a miracle drug that can wipe their memory and allow them to read their story with fresh eyes, without knowing what’s coming, without having rewritten the dialogue and tinkered with every paragraph a thousand times?

When I finally see the finished version, I feel someone’s administered that miracle drug. I meet an eerie stranger.

The day a novel is published is the day it becomes everyone’s – and the day an infinite number of interpretations open up. Other people will make it theirs. And that’s how it should be.

In any metamorphosis, it’s what’s different that matters. A replica is a tame and hobbled thing. But a transformation has wings.

• The 9th Life of Louis Drax is released this weekend.