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 red hoodie.
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 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.
Teresa Palmer mentions in the DVD commentary that her first day of shooting was when she was kissing Perry (Dave Franco) in the car and confessing her love to him (and the camera as it is from Perry's perspective.
The image on the movie poster of R giving Julie flowers never occurs in the movie, but did occur in the book.