A lot of the zombie shrieking was added in at post-production stage. To shoot with numerous actors shrieking constantly would have meant that all the dialog would have been drowned out. Adding it in afterward meant that the sound technicians could manipulate the noise to its best advantage.
The directors were very keen to give the film the additional vibe of a first person shooter video game, where the audience feels personally drawn into the action onscreen. For that reason, the barrel of a gun is in the foreground of a lot of shots.
The 2 in the [REC] 2 movie title most likely represents the 2 submerging story lines between the video camera following teenagers Tito, Ori, Mire and footage of the SWAT team who were featured in the first 40 minutes of the film.
The trivia items below may give away important plot points.
Up until the release of this film, both Jaume Balagueró and Paco Plaza were unaware of a decorative sculpture on the top of the building depicting a monster grabbing a woman by her feet, which is strongly similar to the final scene of [Rec] (2007).