Have to disagree with
dascoyne -- agency was the one thing most missing for me. He's portrayed as a victim all the way through, of mental illness (his mother's and his own), of an impersonal uncaring institution that's supposed to be there to help people like him, etc. Just keeping it in the Bat-verse, Harvey Dent in The Dark Knight had agency. Something awful happened to him, but he ultimately
chose his path. What I most wanted was those pressures leading Arthur to a crisis point where we were rooting for him to do the right thing, even though we knew he wouldn't, where he had a decision gate and could go one way or the other... and deliberately chose to go Dark Side, rather than just getting carried along by external actors. He never "grew up".
You've got me reconsidering some of my initial impressions.
I do believe this character has a degree of agency relative to where I expected the movie to have gone. This film could have been too overloaded with plot mechanisms that function to make him sympathetic. There's an element of that but the trailer led me to believe it would be much worse. e.g. I was expecting a saccharine love story where the neighbor was the only one who saw goodness in him and where a tipping point becomes either her rejection of him or her death. I was pleased that wasn't the case.
My case for the character's (relative) agency is also rooted in elements of Phoenix's performance which, arguably, makes it somewhat subjective. More often than not I see actors struggle with portraying mental illness, particularly schizophrenia. In this case I see an individual who isn't recklessly into his psychosis but credibly portrayed as being in conflict with it. I don't believe he is defined merely by his psychosis.
He does have a transformation but it isn't a singular denouement but is incremental as he systematically divorces himself from the authorities in his life - his case manager, Thomas Wayne, his mother, Murray Franklin ... Granted, it's not a profoundly original story arc but it's one that's well realized, IMO.
This a great film but whether or not this is a great
Joker film is a completely different question.
I confess I hadn't really stopped to consider the second question. This character is compelling to be sure, but I'm now bothered at how much of The Joker's development remains unrealized. I appreciated this film as a depiction of his psychological substrate that would leave a future story to develop his evolution in criminality but, even then, there ought to have been at least some sense of Arthur's intellectual aptitude - not just his psychopathy. If anything this Joker actually seems more of a simpleton. That's not a quality we need to attribute to The Joker.
In fairness he's probably not so much as a simpleton as someone who is just socially inept. But ineptitude isn't something we need to attribute to The Joker either. The most glaring fault of this movie is the lack of any sense of his competence in anything whatsoever. That notion is bothering me more and more even as I write this. This Joker doesn't even have a particular mastery of his own psychological structure let alone anyone else's. Where would this Joker have developed an education, skills, knowledge ... wit?
In summary this is a great movie as a well-realized and engaging character study. I'm tired of actors (e.g. Leto, Nicholson) thinking they just need to be shockingly eccentric, bombastic and menacing. I think Joaquin Phoenix did an immense job with the performance and the look and tone of the film are fantastic. I want to watch this film again as a stand-alone experience. But as a Joker film I have no idea how they could bridge the chasm of character development to get anywhere near where The Joker is supposed to be.