A few tips on directing actors stolen from here and there throughout the years: Rehearse, rehearse, rehearse! Play is important. Rehearsal is the perfect environment to explore and refine ideas and give room to your actors.
Result directing does not work with actors. Result directing means telling the actor what you want to see from them. How he should feel, react or behave. A good actor needs only to know “where” he is going and why his character is going there. Make him focus on the action, not the result of the action.
Identify what is at stake for that specific scene, what is the character fighting for. Keep performances fresh by suggesting different actions to achieve the same goal, or increase/ decrease what is ‘at stake’ in the scene. The least helpful thing you can say to your actors, is ‘ do exactly the same thing again please’
Actors best respond to verbs and especially active verbs. Giving the character a mean to achieve a goal (for example seduce, plead, be helpful) Give the actor the opportunity to really do something as opposed to an instruction such as be sexy, cry as you say your lines, or the result will very likely feel contrived.
Never tell an actor what you do not want to see. This send the wrong kind of information to the brain. If I ask you not to think about a pink elephant, it is very likely you will.
Use the tone of your voice to set the right emotional foundation. You cannot approach a dramatic scene where an actor is supposed to break down in tears if you talk like a coach trying to motivate his football team. Also, the tone of your voice can be a good device to help the actor lower or raise his/her status.
Music can also be a very powerful device when trying to instil an emotion into the actors.
Actors, in order to do their best work, must be working in a safe environment. “Safe” means that only one person will be talking to an actor regarding his or her work—and that is you, the director. All actors will look to you for direction, guidance and inspiration and will expect you to protect them from the comments, criticism or suggestions of everyone else.
Avoid talking too much about what your project/script means rather than your material vision of the project. The actor doesn’t need to know it. He needs to know what are character’s objectives and super objective, as a start; not why he needs to feel this way in the grand scheme of things.
Status play. Playing with the status of a character (is she/he more dominant or submissive?) might help the actor achieve what you want from him. Asking him to raise or lower his status might direct him in the right direction.
In some cases, mainly with unexperienced actors you can ask them to alter some of their physical conditions. Ask them to breath differently, to be more tense, to act as if they were sick with a headache or stomach ache, for example. This will distract them and could provide a different set of options.
Not all the actors need to know everything about the film or scene. You can hide some information from them, in order to create a real sense of surprise or fear, for example.
Remember, all Action is Reaction. Create the right environment and trust that there is a lot you do not know.
Got more time? Check this post on Acting Methods and Techniques