Fair question. With Screen Direction, the classic example in all the textbooks is a cowboy film. If the Cowboys and Indians are meant to be galloping towards each other, then you need to make sure that they actually look like they’re galloping towards each other on the screen. Hence if the Cowboys are heading left to right on the screen, then the Indians must head right to left. And every time we cut to the Cowboys, they must still be heading left to right, and vice versa for the Indians. Otherwise it looks like they’re running away.

Another aspect: if someone walks into a doorway in one scene heading right to left, then when we see them coming out of the doorway in the next scene, they still need to be heading right to left. Otherwise, it just plain looks wrong. The audience won’t usually know why it looks wrong, but they’ll feel that it is wrong all the same – and the editor will not use it unless she has absolutely no other option.

Obviously, there’s rather more to it than this – the need to maintain constant direction between takes, between shots, and between scenes can get quite complex. But you get the general idea. And yes, I know it sounds simple – but you’d be surprised…