Between two actors: when two characters interact in a scene, often they will be scheduled together to allow for a more natural performance of the dialogue. Actor often project their voices differently, and there may be a a considerable difference in volume between them. At this point two things can be done:
If the difference in level is due to over- or under-projection, ask one actor to match the intensity of the other, so that they both sound like they are “in the same room”.
There may still be a difference if one of the actors is a low talker. What must be avoided at all costs is compromising the performance for technical reasons. This is where good fader riding is necessary. The recording mixer must follow the scene attentively and adjust the level for each actor in turn so that the result is a well balanced conversation.
It is important to learn to do this at the time of the recording even if technology permits us to do it in post. The actors and director need to hear a playback of what they have done to be reassured that the take is good. What they hear should be close to the original; a poor or unbalanced recording makes for a tedious and stressful session.
Consistent level throughout the show: when the monitoring system is properly calibrated and the meters are accurate, make a note of what seems a healthy level for the particular show you are working on, and stick to it for the duration of that project. This is an obvious assertion, though often unobserved with projects that span over several days.
It is interesting to note, that when The Simpsons was being dubbed in French here in Québec, all members of the Simpson’s family were at the mic at the same time. The actors would turn sideways so that they could be closer together, and step up to the mic when it was time to speak. This technique was popular among jazz and folk musicians in the days of mono recording.