Be respectful of other points of view.  Expressing your own is fine if you don’t make the other person feel wrong.  If you think you have, apologize.

Don’t yell, and if you do, apologize.

Don’t argue with the director.  Discussion is fine.  Argument isn’t.  If you do, apologize.

Don’t criticize other actors, on or off stage.  If you do, apologize.

Be nice to the technical crew.  They work hard, and they aren’t your servants.  They don’t get much appreciation.  Say thank you.  A lot.

Directors are almost as unappreciated as the technical crew, and they’ve been busting their butt since long before you got involved.  Let them know how much they really meant to the success of the show.

Say thank you to your fellow actors, too.  They are part of the reason why you look good on stage.

Be cheerful, even if you’ve had a bad day.  Put it aside for the time you’re at the theater.

Remember that you have to live with these people for a period of months, and they have to live with you.  Keep all your relationships at the theater healthy and happy!