Dialogue - on trusting performers:

We work in a group-art. 
Rather than being a solitary writer writing a novel, script-writers are not responsible for every last detail. That's what an amateur aims to achieve. We understand that if we provide the setting for a scene, as well as strong dialogue, and characters who live, then, also informed by punctuation,  any actor in the world will be able to decode what we wrote!
Too many directions on the page and the script-reader knows it's a beginner's script, or they're reading a perfectionist - either category is suspect. 
As a writer, trust your descriptions, dialogue, and punctuation to indicate what you intent - that's how you'll capture the reality of your dramatic and comedic world.
Dialogue - Avoiding perfection.
First, here's a wonderful site that talks about the curse of perfectionism, and recapturing the joy of untrammeled creation:

Sure, in your rough draft, set down whatever crosses your mind, and put in directions and all that. Your first pass at a script is for you. Put in what it will take for you to sit back and remember how the scene looks and sounds to you.
And then in your next pass, take out unnecessary directions.  We polish - not to achieve perfection, but to thwart the biggest enemy of a true script.

One of the best things I ever heard about dialogue came from a wonderfully talented hyphenate - the late Linda Griffiths: Writer, director, producer and actor.  She worked in a number of films I produced, and gave me her great advice about dialogue. Wonderful information & I'm glad to pass along my memory of what she said:
 "I like to mess it up. In real life we interrupt each other, leave thoughts unfinished, get obscure, forget what we're saying and... just trail off.... We get phrases wrong, and sometime use bad grammar. We don't always speak logically and rarely use exposition.  We... do mess it up."

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