For any given opera, the rehearsal process begins for the orchestra with full orchestra rehearsals. These are rehearsals with the conductor to learn the music, rehearse it to the point where we know it pretty well and are ready for the Sitzprobe. There may also be sectional rehearsals during this period; these are rehearsals with only certain sections of the orchestra, the first violins or the strings as a whole. These are really useful for making things better quickly.
The Sitzprobe is where the singers and chorus come into the orchestral rehearsal room and we rehearse all together. Prior to this point, there have been numerous production rehearsals which the singers have been attending; these are rehearsals where the director of the show decides what will happen on stage in the show. These are accompanied by piano, not orchestra. There are primary rehearsals in rehearsal rooms, then Stage and Piano rehearsals, which are on the actual performing stage.
At this point the next rehearsals are called Stage and Orchestras. The orchestra goes into the pit, and the singers onto stage into costume. There is usually one Stage and Orchestra rehearsal for each act of opera, and one at the end for a run of the whole show prior to the dress rehearsal. These rehearsals are usually really exciting; the orchestra get to see the set, to start hearing the opera as a whole.
The last rehearsal before the opening night is the dress rehearsal. This is a run, one where there is an audience, but not a performance as such; sometimes we stop and rehearse if things go wrong. Singers may not sing out the whole time, but may “mark” to save their voices. It’s a useful rehearsal as a barometer of how the first night might go, what still needs work, and how the technical aspects might work. It is also generally when we find out how long the performances are, when we might finish a show.