Leaving out those instances where bad acting may be preferable (refer to the ever current rip off alerts thread), it's hard for me to draw the lines between what does and does not constitute a legitimate performance, never mind a "successful" one. Taking the ever-ready example of porn, I couldn't tell you what I more readily get off on - the more professional, the more amateurish, or the more purely voyeuristic.
Whether or not the act seems "forced" is seldom a concern for myself as audience member. When the possibility of damage being done is raised – damage to the performer and/or viewer – well, it is raised. It is there. I mean, I'm as cynical as the next person, and seldom expect much beyond the usual "bad acting", but at least the notion has been brought into play, and it's hard to say how exactly it will play out.
EDIT I meant to add: in the case of the self-abuser, the audience reaction may necessarily be moot - or, any rate, mute. It could be someone I consider a good friend, on stage threatening to kill herself, and I'm not sure how/if/should I act. My reaction may well be very much internalized and not something I would often bear on let alone remark on.
And after the fact it's still hard to say how exactly it played out - seems that different people will often remember the same(?) events rather differently. (I certainly don't consider myself a great recorder of facts, particularly those playing out in live performance venues...)
To put it another way, I'm less concerned with how "forced" things seem than with how "real" the possibility of real damage. Which person is more capable of hurting themselves (and others)? The practiced performer who has rehearsed hundreds of times in hundreds of venues around the globe, or the one-off amateurish hack... no pun intended... who happens to be having a good (bad) day? The performer sets things in motion, for better or worse, and what happens happens.
Talking the more purely voyeuristic, I love when people weigh in with lines like, "What's it to you? I happen to enjoy self-mutilation." And why not? Why shouldn't a noise/pe show be like watching The Nature Channel?
