Dawson Casting: Ryan Phillippe was twenty-five, Reese Witherspoon twenty-three, Sarah Michelle Gellar twenty-two and Sean Patrick Thomas was nearly thirty. Selma Blair, meant to be a freshman, was the oldest of the actors playing the teens.
Method Acting: When Annette slaps Sebastian in the face during their breakup scene, the slap wasn't supposed to happen. Ryan Phillippe and Reese Witherspoon, who were a real-life couple at the time, became caught up in the intensity of the scene that she impulsively slapped him. Sebastian's look of shock was genuine and Ryan even threw up after the scene was finished filming.
Old Shame: Cruel Intentions 2 is this for Amy Adams, though she does poke fun at it.
Amy Adams is now best known for doe-eyed cutie roles.
Romance on the Set: Ryan Phillippe and Reese Witherspoon were dating and got married for seven years.
So My Kids Can Watch: An odd variation, given the film's subject matter. But Christine Baranski signed on because her kids were huge Buffy fans and were ecstatic at the thought of their mother working with Sarah Michelle Gellar.
Two Decades Behind: Okay, so this is based on an 18th-century French novel. But it obviously takes place in the modern day (late 1990s) given the music heard on the (in-universe) soundtrack and the technology present, which makes the elitist and borderline reclusive behavior of the upper-class characters - not commonly seen in American society since the mid-1950s or so - come off as slightly anachronistic. Not to mention Mrs. Caldwell's disgust at race-mixing, which is especially out of character for a sophisticated New Yorker in the late twentieth century.
Sebastian originally had one when Kathryn refuses to sleep with him even after he dumps Annette, going into a tearful, psychotic tirade and even hitting Kathryn. However, it was cut out because it lessened the feeling that Sebastian could truly be redeemed. Interestingly, Kathryn still tells Ronald that Sebastian hit her, making it seem like a lie when it was originally meant to be true. Fortunately, it's completely in-character for her to tell a lie such as this.