Well, to answer your question, typically the musician follows the Director. If they say vamp, you vamp. If they say bridge you go to the bridge. If they say verse you take it to the verse.
But you should be honest and tell them that you only know certain songs and If they don't want to sing those songs then you are not gong to be able to help them with the music.
Also, at least in my church, the directors and the musician pick music together. What I mean is the directors pick what they want, and the musicians make the call if the song is do-able from a musical standpoint. Sometimes a song is complex and you have to work on it a little longer than other songs, so you have to replace it with something else for that week.
But learning songs outside of the keys you normally play in is a good thing. It expands your vocabulary. But you shouldn't be forced to fumble through a song you don't know, but you need to be honest with your directors up front. I personally wouldn't pull the no-show deal on the sunday you are supposed to play because it sets you up as being un-dependable. Talk with your music minister and directors early so everyone knows what the deal is. Maybe the choir could use a track fo some stuff and you could play the stuff that you know.
Also as a musician, your job is to "support" the vocalists. You shouldn't overplay your singers. If they go in, then you go in with the music. If they hang back, then you hang back. Watch your director and follow their lead.
Jlewis
