Too much headroom, is a common mistake made when doing a mid shot of close up of a single character. People tend to centre the face in the middle of the screen. This would seem to be the obvious choice as the face is the focal point. This can throw off the balance of positive and negative space. Although the bottom of the frame will show the persons neck and shoulders the top of the screen will have too much headspace and ultimately distract from the focal point. When shooting one persons close up in a conversation it is important to leave negative space to one side of their head. This creates the illusion or implication of the presence of other people and will feel more natural when cutting between the other person or other people in the scene.
Does the other background action in a frame determine an audiences ability to accept when a camera has crossed the line? If there are lots of extras and lots of movement it may make it more difficult to follow the actions of the main character. If a scene is more static with less distractions and minimal actions it makes it easier for an audience to follow. Less disturbances may mean that the audience have more attention focused on the main character and don’t find crossings of the line so disorientating because they are so absorbed in the action anyway.
Simplistic scenes with minimal shots in number can require shots to be on the same side of the line. If a scene consists of only 3 shots and you cross the line it can be disjointing to audiences. It is not something that will go unnoticed.
So if a director is going to cross the line maybe they should attempt it in scenes that are simplistic in nature and mise en scene but more complex in the amount and diversity of coverage of the scene.