As an audience member for whom it is very difficult to KEEP from singing along with performers I can say that there are some performers and performances that seem to actively discourage sing-along even when they are SAYING "c'mon and sing!".

on the other hand - i've been working ten years or so on trying to encourage sing-along at a faire I go to; shocked the heck out of the performers the first year I went, because they advertised the show as a sing-along but no one did, they didn't expect anyone to,and pretty much had never had anyone attempt to sing along and then i plunked myself down and sang along with every single song! They now have a bunch of "regulars" who manage to encourage enough people that the shows ARE actually sing-alongs.

MMario