Time is often short. Being on time is important. I think the greatest myth about theater musicians is their instantaneous ability to process information. Yes good reading skills are a plus, but there's more to the game than that. Other parts of the game include: awareness, ability to take a cue, patience, and being a good company member.
Awareness encompasses a lot of things. For example, you're in tech and ten people are trying to communicate with each other. This is not the time to noodle away on your instrument. Let's say you have a scheduling conflict. It's a few days before opening. If one is aware of the chain of command, one should approach the stage manager not the composer/musical director. Your cue is coming up, and you're still reading your novel and not watching the director. The singer just jumped three bars; did you go with her? Awareness can also be called common sense.
Musical directors have different styles. The good ones will give you an idea of the tempo in advance. Sometimes they give the wrong tempo and want to adjust it. One's ability to take a cue comes into play. Some cats are nervous and you have sit back off the tempo. Others really mean what they give. Some cats don't want to conduct at all, so you're left finding it yourself.
Patience... The rehearsals are long. Bring something to read or do in your down time. It makes the process more bearable. Crappy food is a must. The bottom line is this. Everyone in the process has to perform together. The actor has to make his quick change; the sound man has his cue; the crew needs to move the walls; and so on. Tech is the time when all areas of the team work though the show together.
A good company member is someone you don't mind being around. He initials the sign in sheet. He's on time and ready to play. He has a pencil and marks his music well. He takes his book home and practices. He's reasonably positive and has a decent attitude. A good company member doesn't comment on areas outside his expertise. i.e. ( A musician giving actor notes. )
Notes are method of improving the work, suggestions of how to do something better. Director's give actors notes. Musical directors give musicians notes: softer bar 33, or you missed an entrance etc. If you make a mistake and receive a note it's o.k. Don't argue with cat. If you don't understand something ask about it. Otherwise, be aware of the note and perform better the next go round.
Theater is a great form of storytelling. It's live and compelling. The audience is the unknown cast member. The pay is steady, and you can swing in some cases. The audiences listen to you and occasionally they'll follow your work. I will visit this topic again from the designers point of view.