1. If you accept a part, honor that commitment. Stay with a show until the set is struck after closing night.
2. Shower, brush your teeth, use deodorant and wear clean clothes.
3. Learn your lines on time.
4. Project your voice.
5. Pick up your cues.
6. Don’t direct your fellow actors.
7. Don’t worry about the inadequacies of your fellow actors, just worry about yourself. If another actor cannot memorize his lines, you must know yours so well that you can cover when he screws up.
8. Don’t be negative. If you think the show you are in stinks, keep it to yourself. Your grousing will not make a bad show better, but might make it worse.
9. Give your part some thought? This rule applies only if you want to be a good actor; if you’re satisfied with mediocrity, you can stop at #8.
10. Men, don’t sleep with more than one actress in a show. Actresses compare notes in the dressing room.
(Okay, #10 was a joke.)
These are just the basics that must be mastered before one can move on to more advanced practices. The more advanced the rules, the less they are rules and the more they are guidelines or even choices. But #1-8 really are rules that cannot be gotten around.
Which rule do novices most often break? #4. Actors want to feel natural, not phony, so beginners talk the way they talk in real life. Onstage, even in an intimate theater, an actor must project his voice so the audience can hear it. This feels unnatural to the beginner; he must make a conscious effort at first to talk loud. The experienced actor has projection automatized in his subconscious, so that he feels unnatural if he does not talk loud.
The second biggest problem is #5, picking up cues. A play is not a real life dialogue. If there are big pauses before every line, then the play drags. Each actor must begin speaking as soon as the previous line ends, unless a decision to pause has been made. This is not natural, so the novice must make a conscious effort to pick up cues until it is automatized in the subconscious.
UPDATE: A director I know suggested two more rules:
11. Take direction -- that's what it's there for.
12. The proper response for a note from the director includes only two words: Thank you.
UPDATE II: Regarding those last two rules, I should note that directors can be wrong. No human is omniscient. For instance, I've had directors tell me to cross the stage when it makes absolutely no sense to cross, there is no need for the cross, the cross actually weakens the scene and I should just stay where I am. You know what I do in these cases? First, I follow rule #12 and say "Thank you." Then, after a day or more has passed and the director has forgotten his lapse of sanity, I ignore the direction. Usually, if what I am doing makes sense, then the director forgets his bad direction and it is never heard of again. If the director brings it up again, then we must have a discussion. If he insists, then I do what he wants and keep my complaints to myself. The director's vision is the artistic integrator, and in the end he is the boss.
I guess I'm saying that rule #11 is not quite an absolute rule like rules #1-8 are. For beginners, it is good advice, though, as the chances are greater that the director is right and the beginner is wrong. Problems can ensue with actors who have a little experience and think they know more than they really do or actors who are just stubborn, defensive, lazy or stupid. But then, those people cause problems in any endeavor, don't they?