I was attending a choir anniversary yesterday and I lead a song for the choir but the musican was playing in a different key from what I was singing in, I can't see why he didn't hear it, I had ask him to stop playing. The musican there who was there I could see was maybe above intermediate in his playing, but when the other choirs also was singing he was playing in the wrong keys, from what they were singing in and several choirs asked him to stop playing and he would not just kept playing, now anyone knows who sings know if you are singing in one key the musicans is playing in another key it is a struggle...not to mention sounds bad too...do you think it is rude to ask a musician to stop playing.
No, not at all.
I can remember one time, I was at an engagement, as a director, and the choir was singing a song a cappella and they were in the wrong key (we didn't have a keyboard player).
Well, I stopped the choir; went to the keyboard and gave them their correct pitches; and restarted the song....
The pastor at that time appointed me as the MOM a week later.
Excellence is not an option, it's an opportunity to get out of the way and allow God to use you as His vessel. One must be willing in order for that to happen. You can't be a vessel singing in the wrong key, worried about what you or the musician are doing.
Just my opinion.