Several tricks were used to create the right sound effects for some of the gorier scenes. Biting into sausages was used to replicate biting into skin and flesh, and drinking yogurt was used to sound like drinking blood. The sound of the children blinking was made by the skin of a grape rubbing against each other in an almost "blinking" motion.
The title of the film (as well as the novel upon which it was based) refers to the fact that, according to myth, vampires must be invited in before they can enter someone's home (this is shown in the film when Eli asks Oskar to invite her into his apartment). The English-translated title of the book and film, "Let The Right One In," is taken from lyrics to the song "Let the Right One Slip In" by Morrissey.
Eli was portrayed by three different actresses. Lina Leandersson portrayed the child Eli while Turkish voice actress Elif Ceylan provided the character's voice. The elderly Eli was played by Susanne Ruben.
In the bed scene with Oskar and Eli, she plays a game of "how many fingers am I using" by tapping on his back that seems somewhat out of character for her. In a deleted scene which was apparently intended to precede this scene, Oskar plays this game with her and she does not understand it (although she does correctly guesses the number of fingers). Her version of the game appears to be using different words, as if she can not quite remember what it was that he said.
Eli asks Oskar if he would still like her if she was not a girl. This is not only a reference to her being a vampire, but to the fact that, in the book, Eli is actually a boy who was castrated at the same time that he was turned into a vampire. This is briefly referenced in a scene where Eli's scarred genitalia is shown.
Late in production, it was decided that Lina Leandersson's voice would be replaced by a darker voice, because the character of Eli was supposed to be an androgynous boy who had been castrated, and Leandersson's voice was considered too delicate and feminine to depict that. Elif Ceylan was chosen for her less feminine, more androgynous tone. Sound designer Per Sundström stated that her voice also made the character more threatening.