The film follows Arthur Fleck, a failed clown and aspiring stand-up comedian whose descent into mental illness and nihilism inspires a violent countercultural revolution against the wealthy in a decaying Gotham City.

As the show goes live, Arthur tells a morbid joke, confesses to the subway murders, rants about societal neglect of the downtrodden, and berates Murray for ridiculing him.

[46] According to Tatiana Siegel of The Hollywood Reporter, Scorsese considered directing Joker before Phillips was chosen, though a Warner Bros. source said he only became involved because the film needed a New York City-based producer.

[47] According to Kim Masters and Borys Kit of The Hollywood Reporter, Jared Leto, who portrayed the Joker in the DCEU, was displeased by the existence of a project separate from his interpretation.

Masters added that Leto's irritation was what caused him to end his association with Creative Artists Agency (CAA), as he believed "his agents should have told him about the Phillips project earlier and fought harder for his version of Joker".

[6] It took him some time to commit to the role, as it intimidated him and he said "oftentimes, in these movies, we have these simplified, reductive archetypes and that allows for the audience to be distant from the character, just like we would do in real life, where it's easy to label somebody as evil and therefore say, 'Well, I'm not that.

[53][54] In their script, Phillips talked about how difficult it is for patients to reveal their diagnoses, referring to a line from the film: "The worst part of having a mental illness is that people expect you to behave as if you don't".

[50] However, they did try to retain the ambiguous "multiple choice" nature of the Joker's past by positioning the character as an unreliable narrator—with entire storylines simply being his delusions[40]—and left what mental illnesses he suffers from unclear.

[41] $25 million of Joker's budget was covered by the Toronto-based financing company Creative Wealth Media, while Village Roadshow Pictures and Bron Studios each contributed 25%.

[119] He sought to portray a character whom audiences could not relate to and did not look to previous Joker actors for inspiration; instead, he read a book about political assassinations so he could understand killers and their motivations.

Arthur's costumes, designed by Mark Bridges, were matched to look "aged, overdyed and distressed" through wearing him with cheap polyester pants and an acrylic sweater.

[125] In the first scene, Arthur's "too small" hooded jacket and white socks emphasizes the childish concept, being a reflection of him living with his mother Penny, with her calling him "my little boy",[124] and the influence of Charlie Chaplin.

Additionally, his suit colors (green, yellow, purple and red) were contrasted with those of his antagonists, including Thomas Wayne, giving them to gray and blue like Batman.

[144] Joker had its world premiere at the 76th Venice International Film Festival on August 31, 2019, where it received an eight-minute standing ovation and won the Golden Lion award.

[151] On September 18, 2019, the United States Army distributed an email warning service members of potential violence at theaters screening the film and noting the Joker character's popularity among the incel community.

[167] In terms of budget-to-gross ratio, Joker is also the most profitable film based on a comic book,[168] due to its small budget and little decline in week-to-week grosses during its theatrical run, surpassing the record previously held by Deadpool (2016).

The site's critical consensus reads, "Joker gives its infamous central character a chillingly plausible origin story that serves as a brilliant showcase for its star – and a dark evolution for comics-inspired cinema.

[193] Mark Kermode of The Observer rated the film 4 out of 5 stars, stating "Joker has an ace card in the form of Joaquin Phoenix's mesmerisingly physical portrayal of a man who would be king".

[194] Writing for IGN, Jim Vejvoda gave Joker a perfect score, saying that the film "would work just as well as an engrossing character study without any of its DC Comics trappings; that it just so happens to be a brilliant Batman-universe movie is icing on the Batfan cake".

[196] Variety's Owen Gleiberman wrote, "Phoenix is astonishing as a mentally ill geek who becomes the killer-clown Joker in Todd Phillips' neo-Taxi Driver knockout: the rare comic-book movie that expresses what's happening in the real world".

Though he praised the performances and thought the story worked, Kenny criticized the social commentary and Phillips' direction, finding the film too derivative and believing its focus was "less in entertainment than in generating self-importance".

[203] Stephanie Zacharek of Time, in a negative review, labeled Phoenix's performance over-the-top and felt that while Phillips tried to "[give] us a movie all about the emptiness of our culture ... he's just offering a prime example of it".

DC Comics chief creative officer Jim Lee praised it as "intense, raw and soulful" and stated that it had remained true to the character despite deviating from the source material.

I don't think anyone would have looked at that material and thought, 'Yeah, let's take [Taxi Driver's] Travis Bickle and [The King of Comedy's] Rupert Pupkin and conflate them, then trap him in a betrayal of the mentally ill and trot it out for a billion dollars'".

[195][199][228] Stephen Kent, writing for the Washington Examiner, described Arthur Fleck as blending shared aspects of mass shooters and interpreted its message as a reminder that society is riddled with men like the Joker.

[227] Writing in People's World, Chauncey K. Robinson said the film "walks a fine line between exploration and validation" of Joker's character and is "ultimately an in-your-face examination of a broken system that creates its own monsters.

"[232] Jim Geraghty of National Review wrote he was "worried that a certain segment of America's angry, paranoid, emotionally unstable young men will watch Joaquin Phoenix descending into madness and a desire to get back at society by hurting as many people as possible and exclaim, 'finally, somebody understands me!

[235] Psychiatrist Kamran Ahmed highlighted the factors in Arthur's childhood such as parental abuse and loss and family history of mental illness in the genesis of his condition.

[249][250] Ukrainian boxing champion Oleksandr Usyk wore a suit resembling that of Joker in a pre-fight press conference leading up to his fight with Anthony Joshua.

[97][257] Joker: Folie à Deux, with Phoenix and Beetz reprising their roles and Lady Gaga joining as Harley Quinn, was released on October 4, 2024, to negative reviews and a poor commercial performance.