What is it?
It is a rule that establishes a line of action, the camera is then placed on one side, and the following shots must be on the same side, inside the 180 degree arc.
How action line is established?
The action line is typically established either between two actor’s eye contact or in the direction of travel. With two action lines the camera must stay inside both, an example would be a three-way conservation.
It is used for conversations, travel (walking, car, etc) or other scenarios, e.g. a fight scene.
Why we use it?
Once an establishing shot is used, the audience is orientated with the actors and the conservation/other action, the 180 degree rule maintains that orientation, as during the shot reverse shot sequence the actor’s eye contact is always facing each other, this creates the illusion of a fluid conservation. If the 180 degree rule is broken the actor’s eye contact is no longer facing each other, therefore they no longer appear to be looking towards each other. This is the same for when the 180 degree rule is used for other action like walking, if it is broken it can appear that the actor is walking in the opposite direction, disorienting the audience.
Effects of breaking the rule?
The rule can be broken for dramatic purposes if the story supports it.
It could be broken to create a disoriented feeling, useful if a character is confused or lost.
What can shift line of action?
Movement of head/eye contact.
Change of positions.
Camera movements that show the camera breaking the line, allows audience to re orientate.
Neutral shots feature the camera on action line, allowing it to move on either side of the line without disorienting the audience.
Cutaway shot shows main action, e.g. conversation, then cuts away to shot of surroundings on opposing side of the action line, the camera then stays on that opposite side.
The Matrix Reloaded Chateau Fight scene breaks the 180 degree rule for dramatic effect using a change of positions in the form of a kick.
The movement that allows the shift in the action line is the kick, as it is what the viewer is focused on when the shot changes to a view just behind the kick(see below) the viewer can immediately re-orientate themselves.
The shot has changed to a point beyond the previously established line of action, the audience is re-orientated due to the shift being done with the action taking place.
A action line is established through the body positions of the two characters facing each other, following shots are taken on this side of the line until it is broken again. This is also an example of match on action.