Preparing to play in a holiday cantatata.

I will be accompanying a church choir in a modern arrangement within the 
next few weeks.  Although I have a tape of it, it is hard to follow and 
there will be only one rehearsal with the orchestra before it is done 
for real.  What is the best way to practice this?  While I want to give 
it my best shot, I don't want to do too much in areas where the brass is 
playing for all their worth and the choir is singing can belto.