Bring something quiet, like a video game (w sound off) or a book for when you're not being used on stage. Better yet, use the time to learn your lines/music.
Always tell the SM or ASM if you need to you leave the room for any reason. Always.
You will get breaks. Please stay present until then, and please use the breaks when they are called. They will vary in length, but please honor the time allotted, whether it's 5 minutes, 10 minutes or an hour.
Be on time. In theater, that means at least 5 minutes before your call. And expect to stay til the end of your called time.
Do not bring cameras into the room. You will also not be allowed to use your phone's camera function. If, on a break (and only on a break), you get give specific permissions from people whose photos you wish, you may take their picture, but if they say no or are not asked, photos are not allowed.
Bring a sense of humor and patience to this process. Things take time, lots of time, to put together. Think of it like a theme park ride, that long wait in line rewards you with a few moments of sheer delight.
Notes only come from the director, musical director or stage manager. No one else should tell anyone what to do. If you have an idea or a concern, take it to your stage manager or director and let them deal with it. And when you are given a note, don't argue, just say thank you. Just that - thank you. And nothing else. If you need clarifying or to understand how to accomplish it, grab the director after notes or tell the SM, and they will help you.
DO NOT move/touch/play with props or costumes or set pieces. Especially if they are not yours. You will be responsible for replacing any broken/torn props or costumes if that happens when they are not in play.
(I am sure there's more - this is just off the top of my head)
