Multi-Track Mixing for Location Dialogue
http://www.equipmentemporium.com/Articles/stereomultitrack.htm

Ginsburg, Fred
Equipment Emporium
2006

Abstract:

Stereo is rarely recorded as such in the field. Instead, we record monaural sounds and wait until post-production is nearly complete to re-assign these sounds to the audience's left, right, and in-between. Until the film is edited, there is no way to know just where all of the audio elements need to end up. For instance, out on production, it might seem logical to record a car that passes from left to right in stereo, so that you can hear the 'pass by' in your phones whoosh from the left ear to the right ear.