The novel was originally started by Siobhan Dowd who left it unfinished, at her death. Patrick Ness finished the book with credits to her idea.
Tom Holland, who collaborated with director J.A. Bayona in The Impossible (2012), was the stand-in for the monster during filming. He received a special "thank you" during credits.
Screenwriter Patrick Ness reveals in commentary that one of the pictures of a young Grandma is actually a publicity photo of Sigourney Weaver from Ghostbusters (1984).
The screenplay for this film was featured in the 2013 Blacklist; a list of the "most liked" unmade scripts of the year.
The movie follows the book very well, with little differences. However, some additional scenes were filmed exclusively for this movie, including one that is set right after where the book ends.