180 Degree Rule

This diagram shows the axis between two characters and the 180 degree curve in which cameras can be set up (green). When changing over the green line, the characters switch places on the screen, this disorients the audience. The 180 degree rule is a basic guideline in film making that states that two objects, be them characters or items, in the same scene should always have the same left/right facing to each other. If the camera crosses over the imaginary line connecting the two subjects, it is called crossing the line. The next shot, from the opposite side, is also known as a reverse angle camera shot.

For example, cutting in two people sitting side-by-side in a moving car talking. To start, the backgrounds are going in opposite directions naturally for each person in the side angles, but you could also cut from looking through the front windshield to looking from the back seat (basically crossing the line) and no audience member is going to be confused as to who is talking to who.

As another example, in The Lord of the Rings: The Two Towers, by Peter Jackson, Smeagle has a conversation with himself which is actually his other personality. Because the filmmakers use the 180 degree rule, and have the "good" Smeagle looking left as he speaks while the "evil" Smeagle looking right, the audience perceives Smeagle as two different characters talking to each other. This is a technique I'd be inspired to use in our thriller film as it's a very cunning way of showing binary opposition, good and evil, white and black.

Photo used from: http://en.wikipedia.org/wiki/File:180_degree_rule.svg