When R (Nicholas Hoult) is talking to Julie (Teresa Palmer) outside her house, the scene is referencing the balcony scene of "Romeo and Juliet." R (Romeo) is talking to Julie (Juliet) on the balcony.
In the book, R wears a black suit with a red tie -identifying him as a corporate "zombie" - and has a zombie wife and zombie kids he has to feed. The film ditches all of this to age the character down and dresses him in jeans and a hoodie.
In preparation for their roles as zombies, the cast and extras went through "zombie camp" where they learned how to move and react as walking corpses.
The image on the movie poster of R giving Julie flowers never occurs in the movie, but did occur in the book.
When R (Nicholas Hoult) returns to the airport with Julie (Teresa Palmer) for the first time after the raid at the lab, an automated announcement in the background can be heard, saying, "The white zone is for loading and unloading of passengers only." This is taken from Airplane! (1980) at the beginning of the movie, when Ted first arrives at the airport.