Bill Hader was unaware that Bill Skarsgård can actually move his eyes in two different directions. Hader asked Skarsgård what kind of editing was done to achieve the effect in the first movie. Skarsgård, in full costume and makeup, responded by saying "Oh, you mean this?" and showed him how he can do it naturally, startling Hader.
Bill Skarsgård (Pennywise) has stated that he had more fun on set during this movie because he was actually able to talk to and hang out with his adult co-stars. Skarsgård had minimal contact with his child co-stars in It (2017) so that they would be more genuinely scared of Pennywise once they saw him. He also had to calm them down after each shot.
In the novel, Eddie ponders that he may have married Myra because she reminded him of his mother. In this film adaptation, the same actress, Molly Atkinson, portrays both Myra Kaspbrak (Eddie's wife) as well as Sonia Kaspbrak (Eddie's mother).
(at around 25 mins) In Henry Bowers' room at the psychiatric hospital, there are several pictures of the moon above Henry's bed. In the novel, Pennywise's face appears in the moon and communicates with Henry.
Bill Hader was approached for the role of Richie Tozier based on Finn Wolfhard's wish to cast him in the sequel. Hader, who had never met Wolfhard, was extremely flattered that Wolfhard wanted him to take on the role.
Brandon Crane: (at around 14 mins) Young Ben from It (1990) makes a small cameo as the businessman on a video call to the current old Ben at the architect firm.
Andy Muschietti: (at around 1h 30 mins) Customer at the pharmacy where adult Eddie picks up his prescription at Keene's pharmacy, background left-hand side of the screen.
Andy Muschietti: [use of slime and vomit] There are scenes involving creatures that are slimy and vomiting on one of the main characters as well as one of the main characters vomiting, as in Mama (2013) and It (2017).