Originally Posted by
transmogrifier
In Fight Club's case, the film is expressly driven by Edward Norton's character's narration (he is in every scene), and because he is equally in the dark regarding the big reveal, it justifies both the "deception" on the film-maker's part and the use of the flashbacks to recontextualize what the narrator had told us before the reveal because Norton's character is literally doing the same thing at the same time, and we have been in his head the whole time. But in the Joker, the filmmakers are literally just inserting the flashbacks for the slow kids down the back.