The story is based loosely on Shakespeare's "Romeo and Juliet." "R" (Nicholas Hoult) = "Romeo;" "Julie" (Teresa Palmer) = "Juliet; "Perry" = "Paris;. "M"/"Marcus" (Rob Corddry) = "Mercutio;" "Nora" (Analeigh Tipton) = Juliet's "Nurse" (the character of Nora is also a nurse).
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.
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.
In R (Nicholas Hoult)'s home (the abandoned airplane), it turns out he owns a Blu-ray of the movie Zombie (1979)) directed by Lucio Fulci, often noted as being one of the greatest zombie movies of all time.
The trivia items below may give away important plot points.
The actors portraying the zombies in this film chose not to blink. It was particularly uncomfortable for Nicholas Hoult, who did a few long scenes without blinking. Later in the film (when the zombies become more human), the zombies start blinking.
The scene where R (Nicholas Hoult) and Julie (Teresa Palmer) jump off the ledge into the water seems to portray the tragic ending of the play where both Romeo and Juliet die in each other's arms. However their survival and R's recovery provide the twist to make this tragic play a romantic comedy movie.
The film changes the ending to the book significantly, with Julie eventually biting R when he kisses her. This results in both of them becoming infected with and becoming something new. R does functionally resurect but he and Julie's eyes become gold, indicating they are now the opposite of zombies. This is preceded by the book's ruminations on the nature of humanity's darkest impulses, which the film largely skips over.
The same car is seen in the same angle three times, but each time, the car gets reconstructed a little bit more (the first time right after Julie (Teresa Palmer) escapes in the car and R (Nicholas Hoult) walks in the sun, then when he walks alone in the rain, and the last time after he meets with the group of zombies walking at the sound of "Rock You like a Hurricane").