The word processor file in which Mavis composes her novel throughout the film is titled "pieceofshit.doc".
When they were casting the movie, Jason Reitman asked Diablo Cody what she thought about Patton Oswalt for the role of Matt, and if the writer knew the actor's work. Cody replied deadpan: "Are you serious?" This, of course, because Oswalt was a semi-regular actor on Cody's television show, United States of Tara (2009).
The crew had difficulty finding a Mini with a tape deck, so they had to jerry-rig one in the car they got.
In the movie, Mavis is the ghostwriter of a popular YA book series. In real life, Diablo Cody, the writer of the film, is a big Sweet Valley High fan (another YA book series with a ghostwriter). The book covers in "Young Adult" resembles those of the "Sweet Valley High" books. Cody actually plans to make a "Sweet Valley High" movie.
One of Matt Freehof's dolls is Nite Owl 1 from Watchmen (2009), where Patrick Wilson (Buddy Slade) played Nite Owl 2.
Jason Reitman: [bourbon] Mavis's drink of choice is a Maker's Mark. In Juno (2007), also directed by Reitman and written by Diablo Cody, when Vanessa offers drinks, Juno asks for a Maker's Mark straight up.