Start with clean dialogue.
If you were shooting a scene where three characters are talking in a restaurant, Bernstein recommends instructing the background actors to silently move their mouths and pretend to talk. This way you have a clean sound recording of each character’s dialogue on a totally silent set as a starting point. If in post-production you find that an actor’s line doesn’t sound right, you can rerecord it in the studio and edit it in, a process known as additional dialogue replacement, or ADR.
Capture background sound.
Then you’ll want a separate recording of just background noise in the restaurant, like the background actors’ conversations and the sound of plates and silverware. This is known as an atmos track because it records everyday sounds that capture the atmosphere of the space.
Record the empty space.
Finally, before everybody arrives or after everyone leaves the room you’re shooting in, record a minute or so of room tone, which is the sound of the room when it’s completely empty. This will pick up the hum of lights and other ambient, real-life sounds unique to that room, which will also be useful to your sound mix.
The idea is to have a separate sound track for every different element of the scene’s audio. As Bernstein says, “We control the relationship of the individual volume, the foreground to the background. So if we increase the volume of the foreground, the background doesn’t go up, it stays low. And let’s say outside we want some traffic noises. Well, to make it sound more realistic, we can record traffic separately, bring that into yet another channel, and control the volume of that.”