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.
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.
Some lines of dialogue were improvised by the actors. Much of the Richie / Eddie banter was improv.
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.
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).