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."
Pennywise has only four minutes of dialogue in the entire film.
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.
Jackson Robert Scott (Georgie) is the youngest in the movie and stated that he wasn't as scared of Pennywise as he thought he would be. He said he actually had a fun time doing his scenes, and that Bill Skarsgård was always joking around.
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).