What we do is get a bunch of extras who can run fairly well. What happens is we have them all standing casually in a public place (park/mall) and have the victim walking around. As he gets to a certain point one of the extras starts to follow him. Slowly at first but then gets faster until the vicitm is running. The other extras join in as the victim passes them, creating a huge stampede of people. In other words a fun way to freak the shit out of somebody.