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.
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 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.
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 trivia items below may give away important plot points.
at around 18 mins) While filming the scene where the young fireman falls from the stairs, not a single actor knew that was going to happen, so the reactions of the characters were the real reactions of the actors themselves.
Up until the release of the sequel [Rec] 2 (2009), Jaume Balagueró and Paco Plaza were unaware of the decorative sculpture on the top of the building depicting a woman being grabbed by a monstrous creature by her foot, which is strongly similar to the final scene of this film.
Unfortunately, several of the zombie's detailed make-up can not be closely seen, due to the shaky camera moves. One example is Jorge-Yamam Serrano, who had one ear replaced by a prosthetic after his character got bitten.