That a lot of the characters in this film have the same sense of humor? Olive, her parents, Mr Griffith and his wife, that guy Olive dated that turned out to be a jerk... they all have that casually sarcastic, deadpan type of jokey personality. I don't know if it's just me but I found this took away some of the film's realism, it just came across like the characters were just the vessel for sarcastic jokes.
shareThis is a common feature of storytelling. In fact I once named it the Shakespeare effect in honor of how William Shakespeare's characters all go around speaking in blank verse and making bad puns and waxing poetic.
Tv shows are the worst offenders. On Gilmore girls everyone in the entire town talked in the same ridiculous way week after week; on Dawson's creek even a mugger in one episode talked like everyone else on the show (in an absurdly verbose way).
Tarantino movies are another great example; also Kevin Williamson movies.