DIRECTOR'S CUT / Snapshot of propaganda from a scene in Listen to Britain (1942): Ian Sellar on sound and vision in the wartime propaganda film Listen to Britain

I USED to think movies were about images until I discovered sound. I don't necessarily mean the chat, I mean sounds that turn upside-down, accentuate, accelerate, reverse, caress, intensify . . . the list is endless.

It was the title, Listen to Britain, directed by Humphrey Jennings that first made me feel obliged to listen as well as to look. When the film starts, just about every image you see has sound that is other than you would expect, starting with a field of corn together with the drone of heavy bombers. Two separate worlds are being put together and their meeting propels us into something larger. The important thing for me was that sound and picture could be treated independently: that the sound could be given a distinct purpose and freed of being the mimic of the image.

In Prague, there is a scene where the main characters awake in a field where they have fallen drunk the night before. I wanted the effect of an island separate from the world, like finding yourself on the moon. The image side of things is quite crude; they are shrouded in mist. But more important is the sound. The mist is there to create a space in which anything could happen. The first thing we have is an unreal silence; I wanted a silence that was slowly filled with the slow rhythmic beat which, as the scene develops, is revealed as coming from a diesel engine in a boat travelling up the river towards them. In fact, the only real sound is that of a cow who was woken up by all the smoke and gave a distant wheeze. It sounded most unearthly, so I kept it in.

The reason for all this is to take the audience inside the characters' experience, which is the only thing that has to be real about a movie. I don't know what Jennings would make of all this. What I describe is not necessarily what he was doing in Listen to Britain, but he handed me a world that I did not even realise existed and I would like to thank him.

(Photograph omitted)