While speaking to Vulture about the movie's milestone, Waters revealed that the he had to switch around a few characters to come up with the perfect bad-girl, good-girl chemistry between the actors. Rachel McAdams for one, originally auditioned for the lead role. And while Water recalls her audition was good, he decided he couldn't cast her as Cady because of her age.
"I remember watching her do the scene," said Waters, "and after it was over, I told her, 'I think you're a movie star, but you're way too old for this character. You just aren't going to be able to play the ingenue.' And she said, 'No, I understand, I get it.'"
But later, when the team decided to have Lindsay Lohan play the lead -- she had previously auditioned and been cast to play Regina George -- casting McAdams as her tormenter "made perfect sense."
"When Lindsay was acting with Rachel, she got very shy, because Rachel was older and a very accomplished actress," said Waters. "She'd come in the room and not talk to Lindsay -- she was very focused. Lindsay kind of got nervous around her, and I thought that, more than anything, was going to be the deciding factor, the fact that she affected Lindsay in that way."
Waters also revealed that Amanda Seyfried was also considered for the antagonist role.
"[Seyfried] tested for Regina and was kind of brilliant, and very different than Rachel's approach. She played it in a much more ethereal but still kind of scary way. She was more frightening, but oddly, less intimidating," he said.
Waters credits producer Lorne Michaels with casting the In Time actress as the "dumb girl."
"So she came in and read it and nailed it, and we got the best of both worlds," he said.
Other shockers from the film include that the iconic four-way phone conversation was shot at different times; that Paramount was hesitant about casting Tim Meadows and Amy Poehler; that the MPAA wanted to give Mean Girls an R rating and that Amy Poehler had a big hand in Kevin Gnapoor's talent show rap.
Waters also dismissed rumors that Scarlett Johansson ever auditioned for a part in the film.
2014: The Year in Music [PHOTOS]