WARNING: The following article contains spoilers for Joker, in theaters now.
Todd Phillips' Joker may be based on characters from DC Comics, but many of those elements are drastically changed in the film. Much like with Christopher Nolan's The Dark Knight Trilogy, most of the characters' more "comic booky" elements are toned down or excised entirely to make way for the film's far more grounded scope.
One staple feature of Joker history that would have been too far-fetched for Todd Phillips' film is Joker's "venom" -- a toxin the villain has been using for decades to contort his victims' faces to look like him. However, the film does seem to have an arguable variant of it -- whether intentional or not.
A major part of Joker is the "kill the rich" clown movement. While the method and premise are different, the imagery of Gotham being swallowed by a sea of smiles connects it to the source material more than you'd think.
First appearing back in Batman #1, Joker venom is, usually, a type of acidic gas that the Clown Prince of Crime uses against his hapless victims. The gas has the effect of stiffening the victim's face into a permanent smile, accompanied by them laughing uncontrollably. This inability to stop laughing, and the obstruction that is causes to breathing, leads to an eventual death for those exposed.
Along with the permanent smile, the victim's face (and sometimes the rest of their skin, as well) will turn white in later uses of the substance, with their teeth yellowing and lips reddening. Ultimately, the individual who comes into contact with Joker venom will become a disturbing facsimile of The Joker himself.
Non-lethal versions have been used as well, namely in more child-oriented adaptations. This more muted version of Joker venom still has the effect of leaving the victim as a mutilated, smiling mirror of The Joker, at least until a cure is concocted.
THE CLOWN MOVEMENT
In Joker, Arthur Fleck scores his first kills on a subway ride against a trio of young bankers. Having been on the way home from a job as a clown at a children's hospital, Fleck is in full clown makeup when the deed is done. This conceals his own identity, but does identify the killer as having a clown's visage.
Believing the killings to be a direct attack on Gotham City's upper class, Thomas Wayne publicly denounces anyone of a similar mindset as being clowns themselves. This gives rise to a political movement among Gotham's disenfranchised, who wear clown masks while publicly protesting elites such as Wayne.
Joker uses the clowns to his advantage throughout the film's climax, first to blend in with the angry citizens of Gotham City while being chased by detectives, and later to draw strength as the accidental creator of their movement.
PUT ON A HAPPY FACE
While the two concepts may seem completely unrelated on a surface level, the former could easily be seen as a more realistic stand-in for the latter. The Joker's aim in using Joker venom, besides its obviously homicidal effect, is always to recreate his victims, and therefore, Gotham City if spread around enough, in his own image. (Even fish have not been safe from the clown's narcissistic form of branding in the past...)
He also wants to reveal, as was the case in his own life, how it simply takes "one bad day" to turn someone into a chaotic clown. Taking a face full of some mysterious acidic concoction and laughing your contorted face to death would certainly have that effect. Exposure to this substance also mirrors The Joker's more traditional origin story -- falling into a vat of chemicals, giving victims the same mutagenic rebirth as their progenitor.
While The Joker in Joker initially denies any connection with the protesters that his actions have inspired, he eventually comes to embrace them as a wider part of his rebirth. This is a dark reflection of the mission that his mother had given him as a boy, which was to bring joy and laughter to all of those around him.
The clown masks are an easy parallel to someone's face becoming like The Joker's but it's the behavior, however, that metaphorically mirror's Joker venom. The hardship that many of the protesters and sympathizers have fallen on in the film is the more generalized equivalent of that "one bad day," devolving into the same chaotic clowns that Wayne railed against in an attempt to strike back at people like him. This uncontrollable rage and violent ambivalence toward the society that had abandoned them is an exact reflection of Fleck's actions. Thus, this is an alternate version of the uncontrollable "laughter" that Joker venom induces.
Similarly, the movement is born of the same violence and disregard as Joker himself, similar to Joker venom recalling Joker's own acid bath. The ability for Joker to blend in with Gotham's antagonized lower classes -- where he once stuck out -- speaks volumes as to how his actions have fulfilled his "joy spreading" mission. Rather than gas, Joker infected Gotham with something far more potent: an ideology.