Everyone thinks they can tell a joke. Jokes can fall flat because the person telling the joke doesn't have the comic timing right or can't remember the details of the joke.

Arthur Fleck (Joaquin Phoenix) is an aspiring stand-up comic who earns a living as a clown. That's not a description of his personality. He works for a company called HA HAs. He puts on face paint and a clown costume and then visits children's wards in hospitals or hawks signs declaring a sale at a shop that's going out of business.
He lives by the motto, "Smile. And put on a happy face." That's the advice he gets from his mother. That takes a lot of effort because Arthur lives with serious lifelong mental health issues. The reasons for his condition are revealed throughout the film.
Because of a condition that makes him break-out in uncontrollable laughter when he is stressed, Arthur finds it difficult to stand in front of an audience and deliver his jokes. The other difficulty is that he can't write funny jokes, despite studying jokes told by television stand-up comedians.
His favourite television comedian is late night show host Murray Franklin, interestingly portrayed by Robert DeNiro, who acted as Travis Bickle, a lonely social outcast with violent tendencies in Taxi Driver.
Joaquin Phoenix immerses himself in his role as Arthur/Joker and delivers a captivating and disturbing portrayal of Fleck's descent into the maelstrom of psychosis.
His physical stance, jagged, boney physique, awkward flat-footed gait when he runs and exaggerated laugh combine to produce a convincing portrait of a man who has internal demons to fight and external abuse to suffer.
The Gotham City cityscape is brooding and suffocating. The soundtrack is heavy to the point of being ponderous. We understand that Arthur suffers anguish and distress without the soundtrack choking us.
The abuse Arthur suffers is reinforced too many times. We get that he is lonely and that cruelty from others is significant in his life.
The film also labours the point of Arthur as the poster boy for civil unrest as a reaction to Gotham's severe crime problem.
Disturbing. That's the word that comes to mind while I was watching Joker and immediately afterwards. Joker is a film that should come with a disclaimer announcing there is a strong chance the content and issues raised regarding mental health, bullying, depression, mental and physical abuse may distress people.
Then there's the sudden, savage and explosive violence.
With Joaquin Phoenix's strong depiction of Arthur Flecks' transition from a person trying to deal with their mental health issues to a psychopathic villain in the Batman saga, Joker is a flawed film with powerful and distressing overtones.