Ezra Miller did his initial audition for the film over Skype and he was so charismatic in the interview that they cast him within 5 hours of the audition.
In the book, Patrick and Mary Elizabeth were chain smokers. That detail was removed in the film, likely to score a PG-13 rating.
In Entertainment Weekly, Emma Watson reported that she took the role because director/writer/novelist Stephen Chbosky told her that "Not only is this going to be one of the most important parts you play, you're also going to have the summer of your life and meet some of your best friends." She also reported that the claim came true.
Stephen Chbosky wrote the book that the movie is based on, he also wrote the screenplay and directed the movie.
In the director's commentary on the DVD and Blu-Ray, director Stephen Chbosky mentions that Dead Poet's Society and The Breakfast Club were two of his favorite films growing up and that they influenced him.
Although never explicitly stated in the film, Sam's full name is "Samantha Dutton" as shown on her SAT results.
Emma Watson has admitted that she refuses to watch her kissing scenes or the Rocky Horror scene.
Unlike in the film, the name of Charlie's sister is never mentioned in the novel.
The shop teacher, Mr. Callahan, is a Vietnam veteran. Tom Savini, who plays Mr. Callahan, actually served in Vietnam.