By the time I started working as screenwriter on the feature film of Room with its director, my fellow-Dubliner Lenny Abrahamson, we already had a starting point: the draft script I’d written after my novel – the story of a five-year-old boy held captive in one room with his mother – was finished and before it was published. But my screenplay went through countless changes, and each one taught me something.

Of all the advice Lenny gave me on rewriting my screenplay, one piece sticks in my mind: he asked me to write the scenes long and loose, “like a wildlife documentary”, and leave the cutting for him to do at the editing stage. I was intrigued by the idea that the audience would be allowed to glimpse Ma and Jack going about their days in their locked shed as unselfconsciously as chimps in the trees, and I sensed that this level of naturalism would really anchor Room’s dramatic storyline.

I remember the moment Lenny first mentioned improvisation, because I was horrified: he suggested I put in something along the lines of “the family chat at the table”, and I balked, and insisted on writing pages of dialogue instead. I see now that as a newbie screenwriter I was focusing rather too much on the dialogue as the one aspect of the script that I’d bet most cinemagoers think of as constituting the whole script. Of course I paid attention to every other aspect of Room as well – the unfolding plot, what scenes to show and how to show them, the descriptions, the characters’ actions – but I know I fretted over the spoken words disproportionately. I tried to whittle them down to those that mattered most (because the paradox is, in film, that sometimes a line can be most powerful if it’s the only one spoken in a scene). I was aiming to get them to a state of high polish: finished.

I’m grinning about that now, because I’ve come to realise that there’s no such thing as a finished screenplay: the document is only a blueprint, and so much will change in the building. You have to allow for – and be prepared to notice, and welcome – what happens on the day, when the cameras are rolling.

When our shoot began in Toronto, I not only got to visit for a day or two every week, but was allowed to see the rushes online every evening. And something clicked for me. I realised that for our child star in particular – Jacob Tremblay was just turning eight – getting him to improvise along the lines of the dialogue was producing much more naturalistic results than making him stick word for word to the script. I found myself deeply enjoying the spontaneous, fly-on-the-wall feel of his interactions with Brie Larson (playing his young Ma).

One of the many unusual things about how Room was made was that it was filmed more or less in sequence. So by the time the other characters came in for the second half of the shoot, I was quite reconciled to improvisation. I realised that the actors were so immersed in the roles I’d written that I could trust them to say the right thing. Some lines came up in rehearsals and got put into the script; others were suggested by the actors on the day.

This style suits a film about parenthood, it occurs to me now, because parents have to improvise all the time. Children’s emotions are intense and ever-shifting, so our response – our makeshift attempts to explain the bewildering world to our kids, and vice versa – have to be just as fast and flexible.

Ma and Jack triumph over their captor by inventing a whole, rich world out of the bare materials he’s allowed them; a magic circle of creativity and delight from which he’ll always be excluded. So I’m glad that the film tells their story in the same spirit, presenting parent-child love as the most powerful of improvisations, a game you have to play on your feet.

• Room is on general release this weekend.