Ability over Appearance: Liz was not intended to be any ethnicity, and her background was not specified in the script. Hong Chau won the part, and a line about Liz being adopted was added in.
Actor-Inspired Element: Hong Chau suggested that Liz have an "unkempt" look and be tattooed.
Awesome, Dear Boy: Brendan Fraser, despite not knowing about the original play nor having seen a script, signed on because he was excited at the prospect of working with Darren Aronofsky. Similarly, Aronofsky was very eager to work with Fraser and was eager to give him a career boost.
Ellie is seventeen, but Sadie Sink turned twenty during filming.
Ty Simpkins was also twenty playing Thomas, who's said to still be in school.
Dyeing for Your Art: It took hours for Brendan Fraser to be fitted with all the prosthetics and makeup needed to bring Charlie to life each shooting day. Fraser also put on some weight to assist in this process.
Extremely Lengthy Creation: It took Darren Aronofsky over ten years to get the film made because he could not find the right actor to play Charlie.
Brendan Fraser went through similar struggles to Charlie after his career declined in the mid 2000s; his wife divorced him, he was in the hospital for multiple injuries that left him unable to do action-heavy roles, and he struggled with the trauma of being sexually assaulted by Hollywood Foreign Press Association president Philip Berk, which led to him being blacklisted by many studios, all of which led to him becoming severely depressed and disappearing from the spotlight for many years. This hasn't gone unnoticed by many fans and reviewers, who have pointed out the similarities.
The film is established to take place in Moscow, Idaho, and a mega-church organization called New Life plays a significant part in Charlie's pain. The real life Moscow, Idaho is the home base of Christ Church which has been accused by residents of establishing a (decidedly anti-gay) theocracy in the area.
Samuel D. Hunter based Charlie on himself, a gay writing instructor struggling with an eating disorder and self-isolating in his apartment.
Real-Life Relative: Sadie Sink's younger sister Jacey plays Ellie in Charlie's memories of her as a kid.
Referenced by...: Saturday Night Live had a sketch where a similar movie called The Hippo had its production shut down. This is bad news for the would-be star (played by the episode's host Woody Harrelson) who actually gained over four hundred pounds to play the main character. It's revealed at the end that the film was cancelled because it was a line-for-line copy of The Whale.
Self-Adaptation: This movie and the play it's based on were both written by Samuel D. Hunter.
Uncredited Role: The actors who play Charlie's online students on the webinar are uncredited.
Wag the Director: Brendan Fraser would only definitively commit to the film if the prosthetics looked realistic on camera. He wanted to show that Charlie was a real human weighed down physically, as well as emotionally, and not have audiences distracted by a static fat suit.
Write What You Know: Samuel D. Hunter based Charlie on himself; he's openly gay, taught writing at an Idaho university, and battled a binge-eating disorder.
which has been accused by residents of establishing a (decidedly anti-gay) theocracy in the area.