I'll be frank, I am in no way a fan of quote origin questions. The main issue I have with them is that it's too likely that the quote could originate out of a type of media that isn't on-topic here. What do we do in that case? Close it as off-topic?
There are two questions that fall into this category that I'm aware of: Origin of knife to a gunfight quote, and the new question Is this quote originally from the dark knight
My feeling is that even if the quote did originate in a non-film media, if it is used in a film, it is still pertinent, as discovering the origin of the quote may yield further insight into the film. For instance, is the Joker referencing a particular character or philosopher?
Insight aside, it also resolves the question of whether a piece of the film's material is original. In my view, inquiring whether a quote is originally from a movie is directly analogous to asking whether the film is an adapted story or original material.
I have to admit that I myself have mixed feelings about my own quote-question, being a rather trivial "where's that from"-question. But then again I still find those things on-topic, as revealing (intentional) inter-references between movies can give good additional insights into the movie apart from a mere identify-problem. But then again, it's probably a question-by-question decision (yes, the unloved standard answer to most meta questions).
And if the original is not from another movie, well, be it so. Other media referenced from a movie can still be relevant for the movie itself ("On which book is this movie based?", "What is the painting shown in this scene and what does it symbolize?").
