As said above, they pull people for "Thank God I'm a Country Boy" at the top of Act 2. I'm pretty sure they only pull from the aisles in the orchestra, so if you don't want to go onstage, don't sit on the aisle or don't look like you want to go onstage, I guess.
"Art, in itself, is an attempt to bring order out of chaos."-Stephen Sondheim