OK - let me give it as I see it. I am (amongst other things) a hypnotherapist. I can see how it
might have been genuine.
As many others have alluded to, hypnosis is all about openess to suggestion. People have varying degrees of suggestibility - there are many measures (Harvard/Stanford
http://en.wikipedia.org/wiki/Hypnotic_susceptibility). I reckon the audience were invited to 'An Evening of Hypnosis with Derren Brown'. The clue is in the title - people came along EXPECTING to either be hypnotised, or see demonstrations of people being hypnotised. So, in the first instance you have a group of people more likely to be compliant to hypnotic suggestion.
Roughly 20% of the population are reckoned to be a 'Hypnotic Virtouso' - these are people who are hyper suggestible . Presumably an above average percentage of Derrens audience were of that persuasion.
There must have been (edited for tv) a number of closely monitored suggestibilty tests done - the hand clasp is a favourite of stage hypnotists. I reckon, after an intense process of elimination, a group of 10 people or so people who exhibited a predisposition for hyper suggestibility, were finally selected.
The acid in the face could have been stooge led as often in those situations when one person sees another person doing it, they follow suit through reaction and compliance.
Eventually when the ice bath was used, just two people were left - the two most highly suggestible - and by elimination one was selected. All the time being watched for hyper suggestible traits by skilled observers.
That one person eventually selected was probably in the 0.01% of the populace.
Subjected to a combination of subtle and blunt post-hypnotic suggestion, Brown worked him over good and proper over a number of days (weeks?)
We dont know how many audiences Brown may have gone through before finding this one person. He was certainly not your average man in the street - he was not even your average hypnotic virtuoso. He was very carefuly selected through a skillful process of elimination.
99.9% of people would not have been suitable candidates for this. Most people will not do anything against their wishes whilst being subjected to hypnosis. However, Brown did the digging until that one-in a thousand turned up. Also, you also dont know how many he chose to shoot Fry until one actually did. Editing is your friend.
On the other hand it could all have been a big con start to finish.