In October 2006, it was announced
Jaume Balagueró and
Paco Plaza would direct the film for
Filmax. Balagueró and Plaza had previously directed the 2002 documentary
OT: la película.
Development and approach Balagueró and Plaza said they set out to tell a classic horror scenario using what they called "the language of television", staged as if a report were unfolding live with a single camera at the centre of events. They cited an epidemic scenario and their affection for zombie cinema as guiding reference points. Velasco recalled that auditions leaned heavily on
improvisation, including being asked to "report" surreal, chaotic situations as if live television, and to play small off-camera presenter beats with the cameraman. The directors said the apartment building became a major creative constraint: the script was finalised in the building and reshaped around what was physically available due to budget limits, including changing a scripted "driving school" meeting point to a textile warehouse found on site.
Filming Principal photography took place in late 2006 in
Barcelona,
Spain. On the audio commentary, Balagueró and Plaza said the opening material at the fire station was filmed like a genuine TV segment at an active Barcelona fire station, using real firefighters, uniforms and equipment; they recalled having no script for the interviews and being interrupted by real emergency call-outs during shooting. The directors highlighted the film's reliance on extended takes, including a continuous sequence they described as lasting "10 minutes or so" with "no cuts" as the camera moves from the fire engine into the building and up through the action. They also described staging select beats without warning some performers in order to capture spontaneous reactions, including a stairwell moment in which "none of the actors knew" a falling body would appear. Balagueró and Plaza said the film was shot in chronological order, which allowed them to review assembled scenes during production and adjust staging and dialogue to avoid repetition as events escalated. They also said the cast were not given the full script and were given a "false ending" during production to keep performances reactive to unfolding events. Plaza added that one on-set rule was to keep rolling—"we would never stop filming, whatever happened"—including through accidental falls and collisions. Velasco likewise said she was not told how the film ended and that some beats were staged without her foreknowledge; for one night-vision sequence, she recalled being told to "react to whatever happens". Velasco said shooting chronologically also extended to costume continuity, with her white top kept throughout so it could become progressively dirtier and bloodied over the course of the story. She described the long takes and repeated stair runs as physically demanding, and noted that the production often filmed at night.
Cinematography Cinematographer said the camera was required to remain strictly reactive—"at no stage could the camera be one second ahead of events"—to preserve the feeling of live coverage; he cited television as a primary visual influence and described treating the camera as a character within the action. He said the building was lit largely from outside the set, including "through a skylight above the staircase", supplemented by practical wall lamps installed by the art department. Balagueró and Plaza said they treated the camera as vulnerable within the story, raising the idea "What if the camera was also infected?" after a blood strike on the lens, and altering exposure and image behaviour accordingly. For the night-vision finale, they said they used an amateur MiniDV camera because professional cameras lacked night-vision capability, transferring the resulting footage to 35mm. Rosso said the set had to be in complete darkness, leaving the actors effectively unable to see while he monitored the action through the viewfinder.
Sound Sound designer
Oriol Tarragó said the film was conceived without a musical score and balanced special effects with what he called a "documentary feel", keeping the soundtrack aligned to the camera's perspective. Tarragó said infected vocalisations were created by blending performer recordings with animal sounds and other sources to give different infected distinct character while maintaining overall consistency. Both Tarragó and the directors described pushing the corridor gunshot to an intentionally harsh, distorted impact to evoke the deafening effect of an indoor gunshot. Tarragó said the night-vision sequence strips the mix back so that "the only thing you can hear is them breathing", and he highlighted the use of off-camera noises to suggest a threatening presence before it is seen. Sound recordist Xavi Mas described the production's long moving takes as "a bit like a dance" between cast and crew, and said the team briefly considered recording sound directly on the camera but rejected it to avoid losing dialogue and subtle details. Mas said the production began with "ten live tracks" and used ten microphones for most of the shoot, which increased the complexity of dialogue editing in post-production.
Make-up and effects On the audio commentary, Balagueró and Plaza said they aimed for the infected to appear sweat-soaked and feverish, using heavy glycerin to give performers a glossy "wet" sheen. They also said the Medeiros make-up was prepared separately so the leads would not see the design before the reveal, and that the night-vision shoot heightened the cast's reactions because they could not see in the dark. ==Release==