Yeah, I remember a New Yorker or similar article a decade or so back watching Springsteen prepare for his tour and him spending the afternoon practicing 'spontaneous' knee-slides. The idea that anybody at that level is really going to go off-piste is madness. Nick Cave's foray's into the crowd are planned and considered to the nth degree. Nothing wrong with it, of course, but lets not pretend it doesn't happen. Look at The Who using guitar lines from earlier shows on the tour. There's too much money at stake, and crucially, too much conservatism amongst the ticket-buying public to allow anything genuinely chaotic. Punters would be on the blower to their lawyers and suing before they'd reached the parking lot.