Here's some fun reading from a 2012 article about how Derren makes his shows:
To get an idea of how his illusions are created, here’s an interview with somebody who featured in one of his earlier TV shows. They wanted to remain anonymous so anything referencing the specific show has been removed but it gives an insight into the way it works. Here’s what he said:
“I agreed to take part because I thought it would be a fun experience. I enjoyed Derren Brown’s shows and it was exciting to be a part of it. I approached it as a kind of acting role; like I was playing a version of myself – rather than it being me.
“And as a piece of entertainment I can appreciate the finished item; it worked well. But it was really strange to see people’s reaction after it was broadcast; how seriously some people took it. They don’t see it as entertainment, they genuinely believed Derren Brown was demonstrating some kind of special power.
“The reality is nothing like what you see on screen. With me, they made it look like I was being secretly filmed during my day-to-day life. It’s something they often do. But that was all planned and pre-arranged with the production team. They told me where to go and what to do. There was a man lugging a big camera around right in front of me – so it was hardly secret.
“When it came to the final scene I was supposed to have been ‘programmed’ to act in a certain way by various subliminal messages and triggers, but that was just part of the misdirection for the benefit of the viewers. The way it actually worked was that Derren was off-camera and giving me directions, telling me what to do.
“Before the filming had started he’d been through a hypnosis routine with me; although this was never mentioned or shown in the finished item. I never felt hypnotised but I went along with it.
“And after a couple of minutes Derren seemed to drop the pretense and switched to more of a work mode. When it came to the filming he was directing me from off-camera – not much different to how a producer would direct an actor.
“It was all slightly disappointing. I’m not sure what I’d expected exactly but I just thought there would be more of a ‘trick’ involved. It blows away the mystique – but it’s the viewer the illusion is being created for.”