Don't read if you haven't seen....
I don't think there much in the film that can be taken at face value, because the Joker is the narrator here, and he has a long history in the comics/films for being unreliable in that regard. Heath Ledger also played on this fact in the last movie with "You wanna know how I got these scars?". The Joker cannot be trusted. Period. The genius of the film is that it is setup to be ambiguous & multiple interpretations that are all plausible, depending on the viewer.
The key part of the movie is when we realize the relationship with his girlfriend was all in his mind. That's a huge moment when he crosses the line & spirals downward into becoming full-on Joker. So what else did we see that was a figment of his imagination? The first time he met Murray Franklin & was called down from the audience? Having a successful stand-up performance after an incredibly bad start? What about being praised like a God in the streets by his worshipers after the car crash? It's debatable if any of those things happened at all.
The main question revolves around his relationship with Thomas Wayne. Was his mom really telling the truth? Did his mother really have an affair with Thomas, which resulted in Arthur's birth? Or did his mother's mental illness cause her to make it all up in her mind? Or did Thomas Wayne use his power to falsify Arthur's adoption records to bury the truth? So are the Joker & the Batman really step-brothers? No matter how many repeat viewings, we'll never know the truth. That's what makes this film so interesting & unique.
And what about the ending in the mental hospital? What was the joke that Arthur was laughing at when he told the therapist "You wouldn't get it"? I think he was thinking of the irony that Thomas Wayne rejected his mother, buried the truth about Arthur, which drove his mother crazy, which led to Arthur's bad upbringing/problems, which led to him becoming the Joker, inspiring a clown movement, which inadvertently caused Thomas Wayne's death. Yeah I think the Joker would find that pretty funny & satisfying to say the least. But at this point of time, it's only the audience that knows these events also lead to the creation of The Batman. That's pretty messed up, and it's the only Joker story I've seen or read that makes this ironic step-brother connection. That also helps to rationalize why after so many years of battling each other, the Joker & Batman are never willing to kill each other. It's a thread that can really be explored in future films if Phoenix comes back to the role.
What about the final final scene with the Joker running down the hall leaving bloody footprints & comically escaping the orderlies? Did you think he just murdered this therapist, running with nowhere to escape? Or was this perhaps another one of Joker's delusional thoughts, representing the fact that his mental illness & all the violent acts that he's committed will always follow him & he can never escape them?
Or perhaps the entire movie is one big joke. We saw him bashing his head against the door of his cell in the beginning of the film. Perhaps Arthur was never released from the mental hospital in the first place, and the whole entire film is his delusional fantasy? Perhaps he's not the real Joker at all, because his face paint doesn't look like the version we know. Perhaps Arthur just inspired the real Joker to rise up?
Again great great great fricking movie where nothing is what it seems IMO. I read an interview where Phoenix said he never thought of Joker as being his dream role, but now he can't stop thinking about it, and how there are endless possibilities to go with the character. IMO this is my favorite Joker performance of all time. Ledger was realy great with his "psychopathic chaos creator" version, but I prefer this version where the mental illness & a real-world take is front & center. Best version of his laugh as well. I hope Phoenix gets to play him again.