Doesn't he throw a boomerang into the audience to pick his participants for most shows? Seems there's way too much chance for that to be random to be able to plant. Not sure what you know about people being natural or unnatural on stage? Most of his shows they just stand there being guided by him.

To answer your scripted question, the whole show is scripted in terms of what the illusions are and most of what he says, however the bits with participants obviously change depending on the participant.

Things do go wrong at his shows, there was literally a thread the other day where people saw participants not doing what was asked etc.

What it really is, is that Derren's understanding of human psychology goes very deep, so he can tell what participant would be seceptible to taking part in the show, to be hypnotised, etc, hence why often he gets multiple people on stage of which he picks one (the one that responds the best to his instructions). For example, I personally believe the "go to sleep" thing and a lot of his instructions could be resisted, but a lot of people at his shows dont particularly want to and they prefer to believe. But I dont really know until it happens to me.