Jack Dylan Grazer (Eddie) was the first one out of all the kids to work with Bill Skarsgård (Pennywise). During their scene, Grazer would cry and gag while Skarsgård was right in his face yelling and drooling. Skarsgård was genuinely concerned for Grazer and after the scene ended, asked him if he was okay. Grazer looked right at him and said, "Love what you're doing with the character!" Skarsgård was left confused and impressed at Grazer's attitude, calling the child actors "little professionals."
Director Andy Muschietti stated in an interview that he specifically cast Finn Wolfhard to play Richie Tozier because he immediately recognized the shared similarities between Wolfhard and Richie in that Wolfhard too, had a burning need inside to express himself and be funny in real life just like the character Richie Tozier does.
Sophia Lillis says that she had to stand on a box in some scenes as, at only five feet tall, she is the shortest of the cast.
The teeth prosthetics that Bill Skarsgård wore as Pennywise made him drool profusely. Director Andy Muschietti liked this as that the drooling adds onto the ravenous nature of Pennywise.
Pennywise has only four minutes of dialogue in the entire film.
Andy Muschietti: In an extended scene where Eddie (Jack Dylan Grazer) goes to refill his prescription, the film's director can be seen in the background for a brief cameo.
Andy Muschietti: [use of slime and vomit] There are scenes involving creatures that are slimy and vomiting on one of the main characters, as in Mama (2013) and It: Chapter Two (2019).