The filmmakers behind Joker have been clear that they never intended to adapt a specific comic with the movie. Now that we can all see the film for ourselves, it's obvious they meant it. Here are some of the biggest ways the Joker of Joker is different from the comics. Spoilers ahead!
Throughout Joker, Arthur Fleck undoubtedly lives in a world of fantasy. As he's watching a late night talk show with his mother, Arthur imagines himself in the audience, eventually being called up to the stage by host Murray Franklin. He has an imaginary relationship with his neighbor Sophie, thinking she's in love with him when she barely remembers his name.
These fantasies call every event in the film into question, making it easy to argue that the movie's climactic scenes are also a kind of fantasy. It's possible to interpret most of the film as simply Fleck's imagination, and that he's always been a patient at the mental institution we find him in at the end.
While the Joker of the comics is certainly no less prone to delusion, we're not seeing DC Comics solely from his point of view. Joker is not just a violent fantasy he's real, and his actions have consequences. While the Joker movie teases the possibility of the plot being a grand delusion of Arthur's, the Joker of the comics is a problem for everybody.
One of the more predictable events at the end of Joker is the slaying of Bruce Wayne's parents. As Gotham erupts in violence, the Waynes emerge from a theater showing 1981's Zorro: The Gay Blade, and Thomas Wayne steers them down an alleyway in hopes of avoiding the riots. Unfortunately, one of the clown-mask-wearing rioters spots the family and follows them. He calls out to Thomas Wayne and tells him he's going to give him what he deserves. He kills the couple, yanking off Martha Wayne's pearl necklace in the process, and leaves the young Bruce Wayne helpless with his parents' corpses. It's possible you've seen all of this before. Keep watching the video to see which ways Joker completely ignored the comics!
What's really happening? | 0:15
Batman's origin story | 1:06
The Joker's real name | 2:23
The Killing Joke | 3:16
A made-up look | 4:29
The nuthouse | 5:29
Hey brother | 6:28
Joker's mercy | 7:34
Gadget-free | 8:17
A "super" villain? | 9:19