Sadie Jones on being both writer and observer

The first television set I visited was in Jamaica in the 1970s. My father was filming a drama documentary, The Fight Against Slavery, for the BBC, and we were there for the summer. My father, who is Jamaican, had written the script and was presenting and my mother, who was an actress at the time, had a small part in it. So he was working and visiting family, and it was a half-holiday for them, and a big adventure for me and my sister. I was six and Melissa was nine. I remember the excitement of being told to stand very still behind a tree to avoid getting into shot. The director was staying with us, and he and his wife used to sunbathe naked, which Melissa and I found hilarious and the Jamaicans thought was very shocking. 'It's not the South of France!', my mother said, furiously.

Years later, as a runner for a small corporate video company, I spent time on set in much less glamorous places. Skeleton crews and no catering. A night shoot in Herne Hill when I was coming down with measles. A stationary shop in Brixton. A lot of coffee and tea making, being bullied, and being very, very cold. Apart from that, I haven't visited sets very often. If one doesn't belong, or have a job to do, it's an awkward thing to be a guest on a film set. Film sets are very rigid, hierarchical places, and though to an outsider they seem vaguely chaotic and formless, everybody has a strictly defined role, the slightest intrusion is noticed, and breaks with etiquette are resented. It is unusual for a writer to visit set more than once, and some don't feel the need to do that. Those who do can earn a reputation for being obnoxious, and it's not unheard of for them to be banned, or at least discouraged. There isn't room for more than one 'author' on a film set. As the daughter of a screenwriter, I knew all this, so when The Outcast was at long last green-lit and ready to go, and Iain (Softley, the director) said it would be all right with him if I came along, I knew I had to tread carefully. I was a stranger and guest in my own story, but grateful to be.

The film had been in development since 2008, and filming started towards the end of September 2014. I drove to set on the first morning, following the instructions on the call sheet, my sat-nav, and the little treasure-hunt location signs tied to trees and lampposts, until I saw the big white trailers of the production base. They were grouped in a clearing like a sterile circus camp. I had no idea where I should go, where Iain was, or the producers, or what I should do. George Mackay, who plays the older, 19-year-old Lewis, was still in the States, shooting a film, and so they had scheduled the childhood scenes, that take place in the '40s, to filmed first. The very first scene was one that had become known as 'the bicycle scene'. It's from the early part of the book, when the ten-year-old Lewis and six-year-old Kit, before tragedy has marked them out, zoom down a steep hill on bicycles in thundery-hot August weather. It was quite cold that September morning as I got out of the car, and it had started to rain. I stood about for a bit in the drizzle, and then I saw the catering truck. It was very early in the morning. I had a bacon sandwich. Then I got a coffee, and was pointed towards the production office. I walked towards it alone, like the first day of school. There was no job for me to do here. I was the dreaded unwelcome set-visitor. I could see the sign on the side of the production office, and headed to it. But my eye was caught by a row of trailers with sheets of paper taped to the doors. I went closer. 'Young Lewis', one said. And, 'Young Tamsin' 'Young Kit'. I saw that there were bicycles; small, reconditioned 1940s children's bicycles, leaning against the steps going up to the closed doors. Inside, I realised, were the children who would be playing the characters from my book.

Writers have strangely formed brains. If the brains of taxi drivers look different to other brains in MRIs, from having developed increased grey matter to accommodate their spatial knowledge, then I wonder if the same is true for writers. Taxi drivers develop their inner eye to see the city streets, writers do the same, to visualise other worlds. We perform this exercise so regularly, so determinedly, so powerfully, that sometimes that other world is more present than the material one. Sometimes, some of us lose sight of which is which. Imagine, then, what it was like to see those bicycles, physical traces of the world that I'd imagined, solidly sitting in this one.

The shoot was nine weeks long, and in that time the crew and actors got used to the disturbing sight of the writer on set and I got used to the disturbing sight of my imagination interpreted. I did not expect it all to be exactly as I had pictured, and of course it wasn't, couldn't and shouldn't have been, but that any of it was there at all was a continuing wonder to me. I did find that there was often a job for me to do, to rewrite or advise, and I did not spend the whole nine weeks in a state of heightened perception, but I will not ever lose that from my mind, how on arriving I saw the children's bicycles.