You are part of the cast, of course, and so are aware that new members arrive and old ones depart. [In the case of the latter, that can be frustrating because they usually leave without permission.]
You have your lines and the others have theirs but there is a great deal of improvisation. The directors must drink heavily because they are seldom around but there always are plenty of critics.
Curtain up and curtain down with little time for rehearsals. If you are lucky you'll know the name of the play and whether it is a comedy or a tragedy. Most of us don't.
Try not to bump into the furniture.