Don’t write dialogue when an action, emotion or image could be used instead.
In writing a screenplay, this is almost always true.
Film is a visual medium, not a verbal one. Sights and sounds must show your story; dialogue should never tell it.
A good test: Read your screenplay without reading the dialogue and see how much of your story you can still understand, and how much of the plot is easily followed. If you can’t understand most of your story and the plot without the dialogue, chances are you’re writing too much dialogue, and you’re using that dialogue to tell the story and the plot.
Go through the material again, this time reading each piece of dialogue while asking yourself what purpose it serves.
Does it tell us something new about a character?
Does it move the story forward?
Does it kick off an emotion or a conflict?
Does it give us essential plot information that can’t be communicated via action?
Can the story survive without it, and in fact is the story better without it?
Why say something when a look, a gesture, a punch, a pinch, a sob, a sly smile, a catch in the throat is more powerful, has more impact, and is more meaningful?
And of the dialogue that then still remains in your piece, is it dialogue that is overwritten?
If you used ten words to say something, can you say the same thing, get the same concept across by using only five words? Three words? And again, better still, no words? Would an image, an emotion, an action work better?
Ideally, dialogue should run no more than four or five lines long per individual character speech? And by lines we do not necessarily mean complete sentences. People don’t naturally talk in complete sentences all the time in every conversation. Neither should characters.
The screenwriting adage for each story element, but particularly concerning dialogue is, “LESS IS MORE.”