The actors were never given the script in its entirety so none of them knew of their character's fates, sometimes not until the day they were actually filming their scenes. This meant that the actors were - more often than not - stressed, nervous and apprehensive on the day of filming, ideal qualities for the film itself.
When it came to casting, the directors deliberately chose actors who were not familiar to help make the piece more realistic, and also actors who were good at improvisation.
The building where the film was shot is the Cedimatexsa building, Rambla de Catalunya, 34 at the Eixample district in Barcelona. It was assaulted by a group of youngsters in 2007, after the film became a hit.
REC was one of the major inspirations for the first person survival - horror game Outlast (2013). The game explores lines between science and religion much like the REC series does with medical science, viruses and possession. Also the POV in the game was also inspired by the shaky found footage style used in REC.
The DVD for this film wasn't released in the US until after its remake Quarantine (2008) had been out in theaters and the sequel [Rec] 2 (2009) had been confirmed.
The directors originally toyed with the idea of having the TV camera crew shadow some policemen. They decided to go with firemen instead as they tend to be more popular and accepted by the general public, especially following 9/11.
Paco Plaza reportedly had Chinese neighbors that never closed their door. This always intrigued him.
(at around 10 mins) Jennifer can be seen holding a camera in the hall. According to the directors Claudia Silva was actually filming but her footage could not fit in the final cut of the film.
Directors Jaume Balagueró and Paco Plaza made a deal prior to shooting in which if one of them pitched an idea and the other one didn't like it, the idea would be tossed out without further explanation.
The working titles were "Bombers" ("Firemen" in Catalan), "Origen" ("Origin") and "Duración: 70 Minutos" ("Lengh: 70 Minutes").
This was released within six months of the similarly themed The Zombie Diaries (2006) and George A. Romero's Diary of the Dead (2007).