Writing is the fastest path to directing, and for those of you with aspirations of directing your own work, understanding the actor/director relationship is paramount to your success. As a director, the most important decisions you will make (and you will make thousands) is in your casting. There is nothing more important than your actors.
Sure, Hitchcock said that the three most important parts of a film are "the script, the script, the script". But even that is arguable. A great actor can save bad material. But bad acting can destroy an awesome script. And because your actors are key, it's important that you embrace that often difficult yet incredibly rewarding relationship.
First off, actors begin by trusting the director. And it's the director's trust to lose. Actors want you to direct them, and a difficult actor is just a scared actor. And why shouldn't an actor be scared? He or she is much more vulnerable than you are - everyone is watching. It's not your face on the marketing billboards, not you on the talk show circuit, and obviously not you on the silver screen.
Movies are more personal for the actor too. Nobody says "I hate George Lucas" or "I hate Steven Spielberg.". They say, "I hate Spielberg movies". But when it comes to the actor, we never say, "I hate Tom Cruise films". Instead, We exclaim, "I hate Tom Cruise!"
Actors are performers - they need to be seen and heard - and you must make sure that your actors feel that you are listening. Imagine: you do a take. It's an extremely emotional scene - a crying scene - filled with tons of subtext and genuine tears. You call cut and immediately make a comment to the lighting guy. Big mistake. Your actor is lost. Your primary concern at that moment is to provide approval and reassurance to your actor.
Remember, you can afford to have mistakes with lighting, sound, cinematography, etc., but you can not afford bad acting. Embrace the actor/director relationship, and you will make magic.