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.
Made and shot in real locations. No sets were built to make this movie.
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.
Filmed chronologically.