The beauty of a large ensemble lies in how everyone pulls together to create something. Everyone is important, even the percussionist buried in the back of the orchestra who has to wait through 100 measures of rest before his one triangle "ping."
Being a part of a large ensemble is an interesting dichotomy. While your individual thoughts about musical interpretation will always come second to the conductor's vision of a piece, you will be sorely missed indeed if you don't show up for rehearsal. You're part of a team.
That being said, here are the rules for being a good citizen in rehearsal:
Show up early. Pulling into the parking lot at 5:00 for a 5:00 rehearsal does not count as showing up on time. Assembling your instrument while the conductor tunes the ensemble does not count as showing up on time. Unless you're set up, warmed up, tuned, and in your chair before the conductor is ready to start rehearsal, you are late. Late arrivals waste everyone's time. A good rule of thumb is to get to your rehearsal venue at least 15 minutes before the scheduled start time.
Practice your part ahead of time. The point of rehearsal is to assemble everyone's parts into a whole. Rehearsal is not the time to work on individual challenges, like tricky technical passages. Also, practice counting through your rests, not just the parts where you play. Coming in at the wrong time because you never practicing counting through a multimeasure rest takes away from time that could be spent working on ensemble-related aspects of a piece.
No talking. There's no reason whatsoever to talk during rehearsal. Most questions can wait until the break, and talking during rehearsal is extremely rude.
Pay attention to the conductor. Be ready to play when he puts up his baton. Stop when he cuts you off. If he asks you to change something, change it. Large ensembles are not the venue for your individuality. Save it for solo recitals.
Stay engaged even when you aren't playing. If the conductor is working with another section, take note of what he says. If he wants the violins to play their melody with a certain articulation, you should use the same articulation when you have that melody. Don't waste time by making the conductor repeat himself.
Know the repertoire. This doesn't just mean knowing which pieces are on the program or how to pronounce composer names. Have a general idea of what's happening in each piece. Do you know when other instruments are playing the same line as you? Do you know what cues to listen for before your entrances? The piece is more than just your part.
And that's a beautiful thing.