Prior to seeing this film, the only character I have seen that comes close to the authenticity of a damaged and dangerous being is Anthony Hopkins' "Hannibal Lecter". But in some ways - ironically! - Lecter is cartoonish in comparison with Arthur Fleck, whose personal history is riddled with misery and pain.
There seems very little let-up in cruelty towards Arthur, except for from one of the characters. It is evident from the beginning of the film that Arthur is emotionally fragile, and mentally off-kilter. He could snap at any time. The build-up to his course of actions follows many incidents of abuse by others. It certainly doesn't help his case that he has an impediment - laughter that he cannot control. This gets him into trouble, at times.
Penny (played beautifully by Frances Conroy) has an undercurrent of sadness and dysfunction that boils over when Arthur learns that she has kept important information from him.
Joaquin Phoenix is brilliant in this role, perhaps his best ever. Arthur/Joker gracefully dances in a few scenes, lost in his own world. In fact, it is possible that some or all of the incidents in this film were not real, but instead the figment of Arthur's imagination. In any event, it was all real to Joker, who reacted accordingly.
Joaquin Phoenix deserves the Oscar and other awards as Best Actor of 2019 for his performance in"Joker", for sure. Artfully directed by Todd Phillips, the movie sets up for the next Batman movie. However - I wouldn't advise viewing"Joker" if you are prone to depression. You need to distance yourself emotionally to some degree, to truly appreciate the performance and messages in this film.