This Harry Potter dementor prank video would make me laugh if I saw this in person. The reactions of the random people on the street are hilarious because they are not associating the 'spells' the dementor is saying with the famous movie. Instead, they are panicking, and being scared away.
I don't blame them, as the dementor is standing right in front of the man, and he has no idea while the surrounding people are fully aware of his existence. When he touches the head of the person being pranked he recites a spell, and the man falls to the ground. He then looks at the remaining individuals, and they usually run away in fear.
I am a huge Harry Potter fan, so I would love to see this in person. Especially because it deals with the dementors, and not the famous cast. The Harry Potter dementor prank was definitely a good scare for those that witnessed it.