The Conversation

The Conversation [Francis Ford Coppola] depicts the downward spiral of Harry Caul [Gene Hackman], who is a covert surveillance expert and private investigator, as his paranoia causes him to get drawn into an elaborate and confusing murder plot. Harry Caul [Gene Hackman] and Stan [Johnny Cazale] are in the workshop editing the surveillance audio from the day prior. Multiple aspects of this scene build up the characteristics that are vital to driving home the overarching significance of what is about to transpire in upcoming scenes to make the movie effective on the whole.
Walter Murch [Sound Editor] and Howard Beals [Sound Effects Editor] managed to edit the audio so that it illustrates exactly what is going on in this scene, and the rest of the movie. Harry begins to his editing session by listening to the raw audio. The audience hears this as a bit of the targeted source audio (the couple) and a lot of background noise. Eventually, as the couple walks in front of a street drummer, you here nothing but the drummer, which prevents Harry from hearing something that may be important. Harry must filter out the garbage audio. He rewinds, you hear the rewinding effects, and plays again while playing with an envelope filter. You hear the audio change as he messes with the faders and knobs of the filter he invented.
This helps communicate to the audience what he is physically doing with the sound without ever taking the time to have the character make an aside and say, “I am doing this right now for this reason.” Harry Caul, as a deeply secretive and introverted person, would never say such a thing to anyone. This character trait is held sturdy throughout the movie and is reinforced again in this scene. There is nothing stopping him from explaining this to Stan, who is curious about what Harry is doing. It is only that Harry Caul is overly protective of his secrets and this builds his character.