Don't tease or mock anyone in a horror film. The monster doesn't like it, and will kill you for it. Not that the monster actually likes the people you mock (it will kill them too). It's just that mocking is the monster's thing. You're butting in.
So don't tease people; don't treat anyone like they're crazy for talking about the monster, and don't play any practical jokes. The last one isn't a [...] [tvtropes.org]