I will have a dissenting opinion of it...
I didn't like it.
The first hour was boring and cynical with a super melodramatic soundtrack. I tend to really get down on a lot of slow movies because while slow burns can be good, they can also go from 'slow' to 'boring'. After a while, Joker tipped into the boring line.
The last third of it did become a bit more entertaining and I really liked the splatter of black humor. I love dark humor and gruesome comedy and maybe the entire film had that but I couldn't tell because the freaking orchestral despairing soundtrack and almost melodramatic emphasis on just how corrupt society is and how the mentally ill are ignored which definitely is an important message.
Yet already the mentally ill have a lot of portrayals of them becoming violent serial killers while in reality most of the mentally ill don't become serial killers.
Spoilers hiding below
Maybe the end of the movie implied that perhaps, Arthur Fleck is merely a sociopath who was forcibly believed to be mentally ill?
But then, why all that emphasis on his mom and what she did to him as a kid? Did he become the joker because of abuse? That makes me uncomfortable...
Do I think this is a pretty good portrayal of a 'Joker'? Not really (needed a lot more of dark Schadenfreude humor).
However, considering it's a side-story, it's...okay.
I will say I do have my own biases. The Joker is my favorite villain for his over-the-top sadism, his clear lack of morals, his crazy sense of humor, and just being unapologetically evil because it's entertaining for him. Since I didn't feel like I got that from this movie, I am being hard on it.
Maybe I'll lighten up if I see it again on Netflix. Maybe I'll see the joke with another viewing.
Right now though? I'm not laughing.
