There's a great thread about opera
here.
A difference worth noting is that often the rehearsal process is shorter than in theatre- maybe 2 weeks of rehearsal before tech. So it can end up being a less "organic" process to figure out blocking, and might be more of the director telling performers where to walk. (Especially when you have a large chorus, children's chorus, group of supers, dancers, or maybe all of the above).
A lot of the blocking, like entrances and exits, is sometimes already determined by the score. You can study the score ahead of time and roughly mark some entrance cues. Opera companies will often rent sets and/or costumes from previous productions, and if the same director or AD is directing your company's production, you might be able to use the paperwork from the previous production to get a leg up on your own paperwork.
Rehearsals are often split into different sessions, and you might have 2 or 3 sessions per day. Not all performers are called to all sessions, so you might have morning and afternoon sessions with just the principals, and in the evening rehearse with just chorus and supers. ASMs will often walk the parts of the missing principals for a chorus rehearsal (or vice versa), so it's important to take good blocking notes.
I second the suggestion of familiarizing yourself with the music ahead of time- listen to it while following along in the score, but also just play it in the car on your way in. It makes things a lot easier if you
know the music. I would also do a little research on the opera itself to see if there are any famous arias (or something similar) that all of the "opera people" in the room will be well-familiar with- you don't want to feel like you're in the dark.
Have fun!
