And the conceit of this section of the film is that, at a certain point, Welles is going to know that he is being bugged. And how this happens is that the action crosses a bridge, and Charlton Heston is forced to go underneath the bridge, in the water of the canal, and the sound of Welles's voice going through the radio is picked up by Heston's recorder, and that sound echoes underneath the bridge. And so when Welles talks, talks, there is a little bit of an echo, echo. And that makes Quinlan, Welles, go, 'What's that, that? Somebody recording me, me?' 'It's you, Menzies, you.' And that's when he realised the turncoat, and the struggle ensues, which results in Menzies being shot. But it's all down to the sound, and not only the sound itself, but the quality of the sound. The fact that the sound, the quality of the sound changed when Heston was forced to go underneath the bridge, underneath this arch, is the thing around which the whole plot resolution of the whole story shifts. So if you're looking for a very good example of how to use sound to help you tell your story in an economical way, there it is. And it's because Welles was a complete director, in the sense of having done radio drama, having done theatre drama, having done film. He knew all of the tricks, and was sensitive to the meaning of something as insignificant as, say, reverberation, echo, and how it could be used at the right moment to help twist the end of the story.
