Filming dialogue is very common in cinema. When we film a dialogue between two characters, there's either an alternation between the two characters, or we see the two characters exchange.
The most classic technique when we shoot dialogue is field against field.
What is a field/counter-field in video ?
A field/counter-field is the filming of two characters interacting with each other. Most of the time, when we film a dialogue between two characters, we'll film one character, then the other. It's this transition from one to the other that forms a champ contre champ.
The 180-degree rule is therefore useful for dialogue between two or more characters, or when they are facing each other.
What does the 180° rule say ? When filming, never cross the line of sight between two characters, in order to maintain spatial coherence between shots. It's imperative to film the scene on the same side of an imaginary line running through the two actors facing each other. (In this case, draw an imaginary line connecting them, the line of sight).
So, to ensure consistency and understanding, and to show that the two are facing each other, the camera must never move beyond this axis. The line therefore corresponds to a maximum camera movement of 180 degrees.
It can also be said that the characters must always be filmed in the same profile during dialogue scenes. Otherwise, it gives the impression that the two characters are not looking at each other, which breaks the illusion of dialogue. By adhering to this simple rule, the viewer cannot be lost in space.