I'm gonna keep it to a top 10 list. Thanks for the challenge @ChrisK
  1. Never be the smartest person in the room
    Pick designers who will take your vision and preferably actors who can arouse a complexity past what you ever dreamed was possible. This is not an excuse not to have an firm understanding of your project; you must know the most so that you can confidently invite more.
  2. Make decisions and be able to live with the consequences
    If you can't do this, do not read on.
  3. Pick a script that means something to you
    Don't do something just because it's popular or because you think it was make money. You spend a duration of your life with the script. If you hate it or fail to understand it, it will show. The stage is the director's dream, don't put on a lousy nightmare.
  4. Then, be brave enough to know what it /really/ means to you.
    ...even if it really is just for money - no judgement, we all gotta eat. Once picking a script that means something to you, it's significance may reveal itself even more - your fears, your losses, your beliefs. Find the courage to put those epiphanies on stage as they come as they are probably very human and therefore the reasons why OTHERS will be interested.
  5. Get a coffee table book of classic paintings and stare at it until it makes sense
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    Composition is not just a city in China... Or something else funny I could say about composition. Understand composition because it will make creating stage pictures much easier/powerful. And before I get shit about letting actors loose to find their own blocking: Theatre is a visual art, there is such a thing as a "strong visual," so kindly usher your actors into impactful arrangements that will make your and their job easier.
  6. Read a lot (high brow and low brow)
    You use your words to direct actors so the more words you know the quicker you can get to a specific action. Absurd, crave, and meticulous are better descriptions than funny, like, or fussy when speaking to actors/designers.
  7. Look at a lot of stuff and note how it makes you feel
    Whether at a museum or a dive bar, notice the elements that inspire emotion. How does a candle in an old soup can with holes in it make you feel as opposed to a candelabra? Where does it place you? Furthermore, knowing the names of major artists/genre's and the aesthetics come in handy and, again, give you a greater specificity when speaking with designers. Hopper, Wyeth, Gothic: all potentially helpful and evocative ideas... As long as you what you mean by them.
  8. Speak in action
    Ask actors to DO things when providing direction, not BE something. "On this next pass, (adverb/ adjective) (verb) at (other character on stage)." AND NEVER GIVE A LINE READING YOU LAZY SON-OF-A-BITCH.
  9. Be very nice to your stage manager
    As a director: you are a jackass and very difficult to deal with. You don't have to apologize for that, but don't pretend otherwise. Be clear when speaking to your SM (even if it's when changing your mind for the 3rd time in an hour) and THANK THEM WHENEVER POSSIBLE.
  10. Know where the rehearsal room stops
    NEWSFLASH: Actors are not your friends and all projects end. Have a life outside of the room. Treat those people in your real life as real people not characters/actors. People in life don't behave /for/ you like characters/actors. It's not your job to shape them, but enjoy them. Excitingly, real people can walk with you regardless of critical success. If you don't accept this or can't sit with a person without a table between you, you will be very lonely with no one to blame but yourself.