It is important when rehearsing to listen to what the director says. It also important to listen to other crew and fellow actors. It is important that you don't just act like a robot, you should suggest ideas, but not too many.
It is important that you learn your lines (see How to learn lines) and your moves if it is dance show. You should learn where you come on and when you come on.
It is important to note when you rehearse that the set will be different on stage and you'll probably have less space (unless you're rehearsing in such a very small space and you'll probably have more space).
It is important to come to the rehearsals that you are called for-keep the information safe.