Screen language
'Screen language' has to do with how the audience 'see' and 'read' a video image. Much screen language is about camera placement, direction of action and where actors need to look. These are important because they give the audience watching your video a sense of 'where people are' in the scene. Screen language can seem a bit tricky, but it's not.
Believe it or not, you are already a sophisticated 'reader' of screen language. You have been brought up with television, video games, films, and DVDs – all of this moving imagery has added to your ability to interpret what is happening on screen. You will know when something is wrong and needs fixing. You will know because you will feel and see something is wrong without knowing quite what – the images will 'jar you'.
Eye lines
For people to look like they are having a conversation when you are editing between two close-ups, you need to make it so they look like they are facing one another. So if Person One is looking left to right in his shot then Person Two is looking right to left in her shot.
Person One
Person Two
If Person One is standing and speaking down to a sitting Person Two then the camera should be a high angle (tilted down) on Person two and then the shot of Person One should be from Person Two's perspective (looking up).
Person One
Person Two
Line crossing
When shooting, camera people and directors draw an imaginary line across their set – the rule is that the camera can only be placed on one side of that line to shoot the shots needed in that scene.
A proscenium arch/theatre analogy works to explain this. Imagine a classic kind of theatre – a stage with the big proscenium arch and curtains.
Now, if you are in an audience at this theatre you can sit at any angle in front of the stage but you cannot sit in the wings or behind the stage. Therefore when you are shooting your scene you should keep the camera on one side of the action or else your audience will lose a sense of the space where the action is taking place.
Of course any rule can be broken but it should be for a good reason and it should 'work' for your said purpose. For example you can 'cross the line' if you do it in shot – so the audience can see where they are going. By this I mean you may track the camera across 'the line'.
Sometimes film-makers have crossed the line intentionally because they wanted their audience to feel confused and unsure of where they were – for example they wanted the audience to 'identify' with a mad character so they made the audience feel like they were going mad.
One should always master the rules before attempting to break them. Breaking rules just to break rules will not impress your audience – it will alienate them.