My mom has always believed that to truly mess with people's heads, you just need to do something that's "a little offside."
If you just stand around at a bus stop, people aren't likely to notice you. If you start jumping and screaming something "random," people might be startled and back away at first. But they'll soon figure out what you're trying to do and either ignore you or tell you to go away.
However, if you quietly start doing a little shuffling dance for no reason like she did once or suddenly spit up ice cubes like I did once, people will stare for longer than even they likely expected to.
And the people responsible for what's going on in these pictures were masters at getting that reaction whether they wanted to be or not.