The basic rule of The Line is that once you establish it, you cannot cross it.
This is because The Line establishes direction of looks, movement and relative positions in relationship to the camera. It makes it possible to maintain a sense of screen direction and where things are even when you change setups and angles.
REDRAWING THE LINE
The Line isn't a permanent thing. You can establish a new Line if the actors move around in the scene, or if the camera moves (say dollies) to a new position, but you need to see the move and establish this new Line.
Hope that all makes sense!
Believe it or not, this rule also applies when editing audio
scenes and using panning. I just got feedback from a producer on a show I've been working on and he noted in one scene that an actor's voice was panned left while her footsteps appeared to move from stage right...busted for a basic mistake. Panning is a great tool, but just like camera movements it must be used sparingly and with proper respect for "THE LINE"!