As awful as this sounds, in my Church work, it's the choir director. I love her to death, but she'll call audibles live when, between the two hymnals, there are close to 700 songs. She'll randomly call a song change, and sometimes with a key change. I thoroughly enjoy having maybe 20 secs to turn to the page, look at it, pull up a combi, and go in some other key. No, we don't rehearse.