This was one of the dead giveaways for me, when he came out that side door, knowing that the "zombies" were inside the compound, he just goes out looking in one direction and does not check behind him or anywhere else. Any normal person thinking they might be in jepardy would only open the door a crack, then very carefully check in all directions before going out. Not standing a few feet form the door whilst they dither about what to do.
Firstly, no one can possibly say what a normal person would do in that situation because no one has been in or seen anyone in that situation, so it's just speculation. Also, at that point he'd probably seen how useless the "zombies" actually were.
Also, another dead giveway was how he remained pretty self concious almost all the way through, but *particularly* when he was asked to tell Derren Brown his name in the helicopter radio bit. He was asked to tell DB his name and he became all self concious, as someone would when speaking to a known celebrity, and knowing they were being filmed, and knowing they definately would be centre of attention at that particular moment. If it was a real situation there wojuld be no reason to be self concious at all at any point, even if you were inherently introverted.
Right, you're a behavioural analyst with a bit of mindreading thrown in...
"I've seen a lot of photoshops in my time and I can tell by the pixels it's fake."