Each cast member had to audition for their parts via online auditions; some were originally considered for other roles within the movie but it was settled that they would play the ones they became credited as.
The character of Andrew was not included in the original script but after Patrick Lynden's audition, the character was devised and inserted into the final draft.
Filming began in August 2010 and continued through to March 2013.
Some scenes, mainly when the characters of Nick and Andrew are alone, traveling to the deserted hospital were improvised.
Many of Nick's lines were improvised by Lee Jerrum.