- Be a vocal coach. Merely teaching the songs to the cast does not qualify as being a vocal coach.
You have to help a singer understand why they're not reaching that note, or why it cracks, or why they don't seem to have enough breath for that phrase, or any of the thousand things that make singing hard. You have to know why and be able to fix it.
- Be a musician. You have to decide what the dynamics should be, make sure that diction is clear and that the ending consonants are perfectly together and make sure that the cast uses the same diction on vowels.
You have to decide where the accents are, and what words matter for the action and decide what style of singing the singers should use (belt, mix, head, etc.).
- Be a teacher. Understand the level of the cast and use the appropriate vocabulary with them. If most people in the cast can't read music, don't start talking about key changes and chord progressions and analyze the music in a theorist way, it will confuse them and you will waste everyone's time.
Rather, use your hands to show where the pitches are, and if the rhythm is tricky, make the cast speak the line in rhythm before adding the pitch onto it.
If your cast is made of brilliant musicians with perfect pitch or great ear training ability, teach quick and don't start singing along everything with them. Get out of the way, and they'll be grateful for it.
- Be a conductor. Sure, when you're in front of the orchestra you can fake your way through, and after a few rehearsals all of it. Simple things matter before then.
You have to be very clear when you count off to start in the middle of song. This is very important as to keep things moving quickly and clearly during staging and dancing rehearsals. Be consistent with what you do.
Singing the being of the phrase and hoping that people will join in won't do it. If you have a pianist during rehearsals, don't "conduct" her by counting out loud every single beat of the measure. Give her the correct tempo to start with, conduct with your hands and don't sing along.
- Get what theater is. You have to put your musician and vocal coach hats on the side and understand that the role of the music and of the singing is to enhance the story. You've got to let singers play with different character voice choices and help them figure out which one works best for both their voice and the story.
You have to appreciate the importance of some of the singing being purposefully not "beautiful" in order to make it more efficient and clear in order to tell the story; know not to fix those moments, because they are perfect theatrically.
You have to be able to offer solutions if music needs to be cut or added, and to know how to do so in a way that keeps on enhancing the story.
- Be available for rehearsals. Some theaters will hire you even if you can't make most of the first weeks of rehearsal, because they think that having a rehearsal pianist there will allow for the process to carry on. It is tempting to say yes to the gig because hey, that's money. Don't do it.
Once you're back, you won't be given the time you need to teach the music because singers will have already kind of learned the songs on their own, but it will drive you nuts that you can't seem to be able to create an ensemble out of them without rehearsals.
You will have missed a ton of important information: when music is cut or added, important change of tempi in some songs, acting and directorial choices that influence the music, etc. The music won't sound good in the end, and you will be blamed for it.
Picture from http://www.flickr.com/photos/thewendyhouse/252987428/