I speak as an inexperienced screenwriter. When I write a book, and someone decides to make it into a movie, I’m glad.

Then? Then the work begins. My first impression is traumatic, as the literary cover is torn off my novel by the screenwriters. It’s a terrible moment: I worked on that text for years, and now everything seems to become impoverished: places, events, characters. A city square minutely described is reduced in the screenplay to the simple common noun: square. An event to which I devoted many pages shrinks, becomes a stage direction. Characters become names, actions are abridged, as are lines of dialogue. Stripped down, the novel suddenly appears to the writer to be a trick of literary words, a fraud, and she is slightly ashamed. The story, in this summary form, is banal. The density I thought I had achieved has vanished. I have to acknowledge that I failed to include things that now seem essential and gave too much space to what now seems superfluous. I want to say, “Let’s give it up – my novel doesn’t seem suitable.”

Then, little by little, one gets used to writing for the screen; it’s a functional type of writing, preparing the leap from the novel to the new work: the film. I calm down: my book is still fine; it contains what I had to write and was able to; it’s sitting on the table, completely self-sufficient. But the film isn’t there yet: it wants to be, and relies on cinematic writing, whose job is to identify the film’s requirements and satisfy them. The perimeter is drawn by my book, but inside that perimeter, everything is rearranged, reimagined based on the show – the real objective. Only at this point is the imagination kindled.

I can see very clearly what, while I was writing the book, was either overexplained or confused. I feel the need for scenes that in my story would have been superfluous. I write dialogue that the tone of my text wouldn’t tolerate. I often seem to be collaborating on “remaking” my novel, with writing that I would never have used. And when it all seems in order – the story and the dialogues flow; we’ve honed, eliminated – the work seems finished.

Yet this is just the beginning, a preliminary goal for writing that, on the one hand, reduces the book to its skeleton, and on the other still displays the features of every written word: ambiguity, openness to multiple representations. In the film or television version, everything, absolutely everything, will have to have a precise aspect: the streets, the church, the tunnel, the houses, the rooms, a classroom, the desks. And everyone, absolutely everyone, will have to have a particular body.

This inevitable definition of every detail will happen outside the screenplay itself. As for the book, it will stay behind, imperturbable, while the film comes closer and closer to one of its possible incarnations.

• Translated by Ann Goldstein

My Brilliant Friend starts on Sky Atlantic on 19 November.