Precisely because it’s confusing, it’s a traditional rule in filming and editing that you never cross that line, unless you have a very good reason for doing so. A good example can be seen in this scene from
'Hunger' (2008), in which Michael Fassbender remains locked in conversation with Liam Cunningham for 24 minutes, with the camera only shifting to closeup over-the-shoulder shots to indicate the intense conclusion to the dialogue. Not once is the line crossed, and it’s not just because it would be confusing to the eye: it’s because giving the characters a fixed visual place helps us attach identity to them, and helps to move the story along by using their positions to juxtapose the characters, their actions, and their intentions.
A terrific example of the line being crossed to good effect can be seen in 'Inglourious Basterds' (2006), in the opening scene between Hans Landa (Christoph Waltz) and the French farmer he’s interrogating. As the scene begins, the farmer can be seen at frame right.
The farmer, Padite, is at the right, indicating that he still has some power in the situation we’re shown. The shot is a faithful obedience to the 180-degree line.
Shortly after this, Landa has realized where the hidden Jewish refugees are and accordingly, the camera shifts the balance of power to him.
Colonel Landa is now at frame right, due to the change in the course of the story. Also note that the light is now on Landa, pushing the farmer to the side and indicating that Landa is now the center of attention.
Because the camera has crossed the line, we know that something must have changed in the story, even though we may not immediately understand what or how. Although the ‘rule’ against crossing the line is often very good advice, you can see why a filmmaker might think of it instead as a ‘180-degree guideline.’
In interviews, as well, it pays to stick to the 180-degree rule. If, in a corporate video, an interviewer begins on the left, and then switches to the right by a director of photography breaking the rule, it could seem as though the interview were being switched around and the interviewer is now the interviewee! This helps to establish a narrative in an interview in which the viewer knows what to expect when the subject of the interview is in shot (a person answering questions) when alternated with the interviewer (questions being asked).
For a thoroughly comprehensive guide to filming interviews click here.