All good screenwriting incorporates the Three C’s whenever possible: CLEAR, CONCISE, and CREATIVE. And it is important to apply the Three C's to all aspects of the screenplay, action, and dialogue.
When writing the slug line...
Be CLEAR. Make sure you are clear as to where the scene takes place (INT., EXT., or I/E.) as well as the specific location and time of day.
Be CONCISE. When describing the specific location, use the least amount of words that give us the most information. For example, INT. ABANDONED WAREHOUSE - CONVEYOR BELT - NIGHT.
Be CREATIVE. Clear and concise writing does not mean to be boring. Describe the slug line with defining details. Instead of just writing INT. APARTMENT - NIGHT, it can be INT. STEVE’S RANSACKED STUDIO - DUSK.
When writing action...
Be CLEAR. Make sure you have complete clarity in your descriptions to what can be seen and heard.
Be CONCISE. In scene description, give the briefest description possible that emphasizes what’s important.
When writing dialogue...
Be CLEAR. Especially when delivering exposition, make sure the information is easy to understand. Remember, character’s are revealed not by what they say, but by what they do and how they react. Make sure there is clarity between dialogue and the actions - and reactions - that surround it.
Be CONCISE. In dialogue, avoid being too talky or excessive exposition. Give your characters the least amount of dialogue possible. If they can say it in a look, let them.
Be CREATIVE. Again, clear and concise writing should never be dull nor forgettable, and this is especially true with dialogue. Your characters must have unique voices - how they talk, their lexicon, speech pattern, their accent, etc. Make each voice original.