HOW DO YOU LEARN A NEW ROLE?
Everybody has a slightly different technique. Experiment; you'll figure out what works best for you in time. Here's what I like to do:
First, I read the score cover to cover, sometimes while listening to a recording.
Then, I translate the sucker. Word for word. Usually I just do the scenes I'm in, and later when I have time and the inclination, I do the rest.
I like to find and read the source material if at all possible.
Finally, I go section by section, starting at the end of the piece, and learn it a note at a time. By the time I reach the first page, it's memorized. This is a slow and painstaking way to learn (and I do cheat when it gets too boring) but it is also very, very thorough.
Once I've learned all the notes and have started to sing it into my voice a bit, I will sing the ensembles along with several different good recordings, to be sure I get the color of the orchestra in my ear and understand aurally where my place is in the ensemble. I also use this to test memorization. You just have to be really careful not to get too attached to any one recording, so that when you're with a live conductor you're able to take his tempi and interpretation without feeling weird about it.
Good luck and have a great time!