People keep saying they would expect a much bigger push and a more dramatic reaction from the pushers - I think the fact there wasn't a big dramatic scream or similar made it more realistic.
If you were apprehensive about shoving someone off a building I'm pretty sure you wouldn't take a running start. There's a huge difference between looking back at a tiny push and a running start shove and surely on a subconscious level you'd be trying to protect yourself mentally. A light push could allow you to convince yourself on some level that he was unstable already, he might have fallen off anyway, he might not fall as hard, you didn't really do much - anything to make it easier to reconcile the act in your head later on. A little push isn't as violent or aggressive as doing it with clear intent and force. It would take some serious dedication to wind yourself up and shove the guy off. It's a very deliberate, aggressive action and if you do that there's no way you can justify isn't to yourself later on. These are just normal people, not aggressive thugs (or at least we hope not!) that believe someone deserves to die. Confused, scared, filled with adrenaline and being bombarded with pressure from all angles. I doubt you'd be thinking 'right I need to put my whole body weight behind this to make sure he goes over, but make sure I don't over balance and go over the edge too so...' Surely you'd be trying to minimise it instead?
I agree that the pushers looked a lot more fake than Chris but I think that's partially down to the fact that we'd watched him all the way through. We'd seen the way he reacts to different situations and been through the story with him. As viewers we were removed from the last three. We didn't have any affinity with them and hadn't grown accustomed to their body language or reactions in the way we had with Chris. On a deeper level, it's a lot easier to think they're fake than to be confronted with the terrifying reality that some people had actually just been coerced into pushing someone to their deaths.
In addition, the only time we've seen this sort of thing happen is on TV. In films the actors all start sobbing after doing something horrific because it's more dramatic visually and frankly that's the reaction you'd hope someone would have. But how many of us have killed someone in real life and therefore actually know how we'd react? I think it would be a lot more likely you'd be in shock after doing it and not thinking or feeling straight. The sobbing would come later when it finally hit you, which is probably why Derren steps in straight away, to stop that from happening or at least to reduce the impact.
Or of course they were all actors and the show was a farce...who knows!