Start late and leave early

Syd Field says that a good guideline for where to start a scene is two lines before the purpose of the scene is revealed. That’s the latest you can start.

The purpose of each scene is defined as knowing what the protagonist of the scene wants. The protagonist of the scene may not be the protagonist of the film. It could be the antagonist or a supporting character.

You should finish scenes as soon as the audience knows whether the protagonist of the scene has either got what they want or as soon as they are denied. Why stay any longer?

If it hasn’t been written that way, editors can fix it in ‘the last rewrite’.