On the DVD commentary track, the actors note that Tim Curry's characterization of Pennywise was so creepy and realistic that everyone avoided him during the filming.
Jarred Blancard (young Henry Bowers) hated having to call Marlon Taylor (young Mike Hanlon) a "ni--er" and would often apologize to him before and after filming for his character's use of the slur. They remained friends after filming wrapped.
Tim Curry was reluctant to take the role of Pennywise initially, because he didn't relish the thought of being buried under so much make-up. When he played Darkness in Legend (1985), it was a difficult and demanding role, and the hours in make-up was still fresh in his mind. To compromise, Writer and Director Tommy Lee Wallace minimized the amount of make-up on Curry, and a lot wasn't necessary anyway because Curry's performance was so strong.
The theater scenes racked up many dull hours of waiting and retakes for the child extras. Tim Curry often came onto the stage in full costume and conducted singalongs with the extras, most of whom didn't know who he was. They thought he was actually a clown hired to entertain them.