Piano Forum

Just random comments.  I don't know the piece.  It looks like piano concerto though.  If it was a concerto, you're the soloist, so you get to make interpretive decisions, along with the conductor.

Ditto on being prepped ahead of time.  The rehearsals are for everyone to make things click together.  You might not be concentrating on what you normally think about during the rehearsal.  I suppose brace yourself for having 50+ people sitting around while you play during a rehearsal.  It's "just" a rehearsal but it's more like deciding and lining things up as an ensemble for a performance, so a performance-rehearsal.  It's also very possible anything that's a long solo for you is just left out for time.  You can play that section, so everyone else doesn't need to sit and wait for you to play through it, burning through the ensemble rehearsal time.  They can focus on the things that require the full ensemble.  Potentially other people, like the more solo vocalists, might not even be there until the last rehearsal or the day of. 

Playing in an ensemble with a different instrument can help.  Conducting can help.  Being able to pick up a beat from someone and hand a beat off to someone helps.

If you're watching the conductor, go be sight, not by sound.  Everything's done by sight since the sound can take a fraction of a second to travel.  Don't wait for the sound.  Although if you're right to the conductor it doesn't matter.  You might hear other people being slightly off which is really distracting.  When in doubt, follow the conductor.  Although if you're the soloist, everyone would follow you.

Ditto on playing with a steady beat.  Even if it's not the style, still practice it with the beat emphasized, so you can do that if needed.  Tap your foot, nod your head.  Make sure someone else can visually determine exactly where the beats are from you while you play.  Essentially conducting at the piano.  If you need others to come in with you at the same time, give an upbeat/breath with your head to cue them.  Be ready to just play like a metronome if you have to.

Study the other parts so it's less distracting.  If you have stretches with rests, practice mentally singing/hearing another part to guide you through the rest so you know where to come in. 

The conductor will be aware pianists work alone and probably don't have every measure number memorized.  They might have you start or have the group start and then you join in.  It's a pain but communication has to be clear for exactly where everyone is supposed to start.  ex. "Five measures before X."

You can make your own "click track" style recordings.  Record yourself on piano.  Then listen to that and record another part (so you learn another part that way a bit).  Then play the piano part again with the recording of the other part going.  You'll see how it is to line things up.  You could also record a conductor "click track" and just tap out the beats, sort of like a conductor.  You don't need anyone else for practice like that.  It's not the same as working with people though.  You're always forced to adjust to the recording.

It looks like you will be following the conductor for some places.  Make sure you can see them.  Go by sight for when you play.  It can also help if you don't have to look at the keys at all in some places so you can watch the conductor (but watch without looking like a machine doing a performance watching the conductor).

It looks like the conductor might not even be able to see all the vocalists.  I'm not sure if the conductor would be watching them a bit or they would turn and watch the conductor a bit.   

When in doubt, keep the beat steady and adjust.  Watch the conductor (unless the conductor is watching you if it's more of a soloist seciton).   Everyone else would be doing the same.  Then you "just" line the beats up together.