Also be sure to highlight tempo changes and time signature changes, those can be crucial in difficult pieces, notably large choruses!
I highly recommend, especially for opera, that stage managers follow the music rather than the singers. Singers sometimes mark (sing lightly), forget the words, and sometimes sing in languages you are not familiar with (also the accompanist has to be the most reliable person you have in the rehearsal process).
I studied music in college and work on opera and I still find I can always learn to read music better whether it's interpreting chords or intervals, or following difficult meters. Good luck!