This isn’t a hard and fast rule for everyone but this was taught to me by Jeannie B.
Every line/beat/scene MUST be able to do one of the three below:
– move the story forward
– reveal something about the character
– infer action
For me, this helps tremendously in he editing process. I love it when every word means something and would help (or hinder) the play if each one did one of the following.
Naturally, this is a take it or leave it lesson. But as the saying goes, in order to break the rules, you have to know what they are first!