Director Matt Reeves modeled the physical appearance and personality of Abby (Chloë Grace Moretz) after seeing photos of a 12-year old homeless girl taken by Mary Ellen Mark. Moretz said the sadness of her character was decided on by her and Reeves after seeing the photos.
Director Matt Reeves explained why a deleted scene, showing Abby (Chloë Grace Moretz) being attacked as a human, was cut. (The scene was released on the Internet.) Contrary to the belief that the scene, depicting her being changed to a vampire and entering Owen's mind, would be too intense for the viewers, Reeves stated that he felt the scene would have disturbed the flow of the film. He remarked that he wished it would have been able to make the final cut.
The candy, Now and Later, is a recurring object throughout the film. Owen (Kodi Smit-McPhee) is consistently eating the candy and hiding the wrappers, the candy's theme song is what Owen is singing in his first and last scene, and Abby (Chloë Grace Moretz) writes her first note to Owen on a Now and Later wrapper.
The speech that President Ronald Reagan is seen giving on the television in the hospital is the famous "Evil Empire Speech". You can hear the line "Reverend clergy all, Senator Hawkins, distinguished members of the Florida congressional delegation..." which is the beginning of that speech.
In a few shots in the movie, a few lens flares can be seen. These shots are most likely a nod to director J.J. Abrams, who not only uses these types of shots in his movies, but he also grew up with the director of Let Me In, Matt Reeves.
The trivia items below may give away important plot points.
Director Matt Reeves stated the main thing he loved about the novel 'Let the Right One In' was the fact that Abby wasn't a scheming 250-year old woman inside a 12-year old body. She was very much a child emotionally and has had to protect herself from getting too close with anyone. He believed Abby was testing Owen to see if he would show he cared for her and stay or if he would get scared and stop seeing her.
Abby asks if Owen would still like her if she wasn't a girl. This is not only a reference to her being a vampire, but to the fact that, in the book, Abby (Eli) is actually a boy who was castrated at the same time that he was turned into a vampire.
In the scene where Abby is bleeding in front of Owen because she wasn't invited in, she wears a KISS Destroyer T-shirt. In the original novel, the KISS Destroyer album was the first cassette Oskar (Owen) listens to after he buys a Sony Walkman.
Owen's voyeurism is a recurring theme in the film. He spies on his neighbors Jack and Virginia from his telescope, his neighbor who exercises at home, Abby and the father from the cracked door, Abby while she is changing clothes, the policeman from the peephole in Abby's apartment, and Kenny on several occasions, including when he is harassed by his brother in front of his friends. Note that nearly all subjects of Owen's voyeurism are murdered by Abby.
In the novel, Eli is actually a boy who was castrated and turned into a vampire. In the original film Eli is androgynous as despite receiving a glimpse of the scarred genitalia implying that his / her novel backstory is canon, it is still left unexplained. In this installment Abby (Eli) is quite clearly a female. A deleted scene even shows Abby being bitten as a child which turned her into a vampire.