Thank You for the Music
The orchestra starts rehearsing rather late in the production, about one month before the premiere. We lesser mortals (who needed month after month to get our chorus parts down) are always amazed at how fast the instrumentalists learn their stuff.
The first time when chorus and orchestra come together, we singers normally forget to sing, because the orchestra does sound so amazing and vice versa some of the instrumentalsist miss their cues because they fascinatedly listen to the sound of the voices for the first time. Then it all clicks and fits together, harmonies which seemed a little off in the piano score we used for chorus accompaniment make perfect sense all of a sudden and you are just blown away by the sound 100 people create together. To me personally, those final music rehearsals are one of the most rewarding things in this whole deal.
I bet you want to know what it sounds like, right now :) I give you a very small glimpse of it here. It is a true bootleg, recorded with an mp3 or md player during final music rehearsals, so the sound quality is very poor.