[He would not be having fun even if he wasn't hauling boxes of books to the library because Elliot Nightray does not know how to have fun.]
It means that you should be careful with what you say. [Not that Elliot knows much about what you should or shouldn't say.] ...but no, you don't have to do anything.
[action]
It means that you should be careful with what you say. [Not that Elliot knows much about what you should or shouldn't say.] ...but no, you don't have to do anything.