Public Stunts: The New York Freeze
Here's a video of a public stunt performed at Grand Central Station last month by 207 members of the Improv Everywhere acting group. They refer to it as a "freeze" where they walk into the train station and all freeze their positions at the exact same moment.
After holding positions for 5 minutes, they instantly resume their actions as if nothing had happened, confusing the hell out of the people who had no idea what was happening.
It's amazing the impact you can have from just a few hundred people doing something slightly unusual in a public area.