That doesn't explain, however, how Derren did the second part of the act, where he was blindfolded and asked those who hadn't written a question down to simply think of one in their mind. Unless that instruction was just to trick us and he was actually still focussing on people who had submitted an envelope.
What made me suspect stooges where used, when I saw the show live, was that Derren called out for someone named Cath and a woman behind me responded clearly. He ignored this until he got a reply from another 'Cath' in the audience moments later.