I had a fun idea in a server once, the admin forgot to disable NPCs, so I hid in a corner and loaded a truck with combine ready to open fire, everyone was god moded, so it was all a bit of fun.
I drove the truck, but alas, there was no gunfight! I checked the truck to see that it was empty. I retraced my steps back to where I spawned them, and saw them standing around. Unfortunatley, the combine had to be put down.
Later, I was running through a map, fighting enemies as you do, and ran across a bridge in hopes my comrades in rebellion would follow, but they did not cross the bridge, and were killed by hunters.
This got me wondering, is it possible to make it so NPCs recognise props as something they can stand on, and could they not fall off a moving prop? It would be very useful in machinima, and would be great for mucking around.