Actor-Shared Background: They worked in Heath Ledger's Australian background to be an actual part of the character, including having Kat ask him if the accent was real or faked.
Dawson Casting: One of the most notable aversions, all of the main cast was between 18 and 20 during the filming of the movie playing high schoolers (Krumholtz was the oldest, and he was barely 21 when the movie premiered). Some of the supporting cast were a little older, though - Gabrielle Union (Chastity) and Susan May Pratt (Mandella) were 27 and 25, respectively.
Old Shame: Averted, a lot of people expected Heath Ledger to continue in the teen comedy genre but he instead refused a lot of similar roles until he started to get the parts he wanted like in The Patriot and A Knight's Tale. Despite this, he didn't consider this film to be an old shame, just not where he wanted to take his career.
Ooh, Me Accent's Slipping: Ledger's accent is sort of half-Amercian, half-Australian. Possibly justified in-universe as he's revealed to be from Australia originally, but to have lived in America for more than a year prior to the film starting.
Romance on the Set: Joseph Gordon-Levitt dated both Larissa Oleynik and Julia Stiles after filming... not at the same time. The main cast apparently became fairly close friends, regardless.
Throw It In: Stiles's tears during the poetry reading scene were apparently unscripted, as was the brief shot of Ledger playing with the Bunsen burner near the opening of the film.