So rehearsing takes a lot of time, and how much of that you'll end up actually doing something is pretty easy to work out. You divide it by the number of people on stage. Well, the chorus tends to be thought of as one mass, or a group of clumps, so the rule doesn't quite hold there, but you get the idea. And in the last Boheme I did each chorus member had their own 'track', their own character with very specific stories to play, and lots of individual onstage business. Needless to say that took a lot of time to rehearse. So in a 3-hour session you will actually be running material for 30-60 minutes. The rest will be notes. You will receive maybe 5-25 minutes worth of these. So potentially you are working for 35min, having tea for 15, and just standing there while others work for over 2 hours. Best case scenario you are still not being used half the time.
OK, I confess, I made all the math up. There are no hard and fast statistics, but the fact is that we do very little a lot of the time. Mostly we have to try to stay out of the way, which is a skill. Standing around is draining, your focus wanders, chatter starts, the room starts buzzing, and over time this wears down everyones patience, most noticeably the director's or the stage manager's, who will at one point probably explode. Student productions, professional companies, it doesn't matter, it's human nature. In many ways it's unavoidable, and at times rehearsal room humour, cracking jokes in the dead time, a bit of banter or even grumbling are simply needed to maintain sanity and any sense of being an ensemble. Still, it's worth practising the art of hanging around without contributing to the din. Once you're in tech rehearsals, where everyones patience has already been worn down to the nub, you want to be as zen as possible.
What can you do? Learn how to just stand their with your own thoughts and one ear open for when something is happening that may have an impact on your 'track'. Play silent games with other people, still with one ear open. Listen to the notes being given, even if they're not for you. If you can sit down, a lot of people do knitting (I kid you not, it's very on trend among opera singers). Practise catching yourself when you are getting a tad loud. Pick your battles, learn to read a room for how much defocusing you can get away with (purists may say 'none', but life tends to disagree). Learn who not to stand next to - either because they annoy you (which is draining), or because you get on too well and tend to default to banter (save that up for the pub or for when it's absolutely vital for maintaining sanity).
Then there are the smartphones. They are everywhere. Rehearsal rooms these days are full of people hanging around hunched over the small screens. On one hand it is something I was told in college is unacceptable, unprofessional, and when I took actual directorial notes on my iPad I risked getting the evil eye... Out here in the profession, all the big role principals do it, and it trickles down to everyone else. Hell, the conductors do it, even some directors. And here comes the other hand - it actually has virtues to it. It's quiet, unobtrusive, passes the time, and if you have a pdf reading app, you can have your score on there for quick reference for those awkward first runs of whole acts where nobody remembers how long there is until the next bit (and which bit comes next anyway?). Yes, there's a whiff of not being present in the space when you're tapping and swiping the phone, and not having it on silent is just plain wrong, but if practiced in moderation is it any worse than knitting or chatting? I don't know, depends on the rehearsal room.
In rehearsals, as in life, the best philosophy is to do whatever you need for you, as long as it doesn't get in anyone's way. Yes, being rehired is often a function of how you conduct yourself in rehearsals, but being the quiet one in the corner who obnoxiously shushes everyone isn't necessarily going to get you anywhere. It's the fun people who are needed and valued, as long as they know when it's ok to be fun.