I think maybe it’s because you feel like you don’t have permission to talk about someone else, or quote a movie, etc. Because maybe you grew up feeling like it was rude to talk about other people. Or maybe you feel responsible to quote them exactly right so you don’t misrepresent them. So this is causing pressure. Also, I believe stuttering is kinda related to identity. So maybe your brain isn’t familiar with you operating from the words of a person with a separate identity than you. So your brain detects their quote as an error and shuts it down. And as far as jokes, I think sometimes we get so excited about getting to the punchline of the joke, or the conclusion of the joke, so people can understand fully and laugh, we feel overly excited. The excitement causes us to want to hurry and express more words than we possibly can per second, which causes our brain to clog up. So you need to enjoy the joke one part at a time, staying in the moment.