In the scene where Cady was asked if her "muffin was buttered," the line was originally going to be, "Is your cherry popped?" The same went for the girl who "made out with a hot dog"; this was going to be "masturbated with a hot dog." These were omitted in order for the film to gain a PG-13 rating instead of an R.
Tim Meadows broke his hand before shooting and had to wear a cast, so the explanation that his character, Mr. Duvall, had carpal tunnel was added.
Initially, Lindsay Lohan was cast as Regina, but she decided to play the "nice girl" (Cady Heron) so the public would not base her real personality on Regina's. Rachel McAdams was chosen to play "mean girl" Regina George because "only nice girls can play mean girls," according to the producer.
Tina Fey envisioned backgrounds for all of the characters. If any of the actors had questions about their characters, she could fill in the blanks for them.
Although they play mother and daughter in the film, Amy Poehler is only seven years older than Rachel McAdams.