There were two prosthetic makeups for Glenn Howerton. The first, which took three hours in the make-up chair, was for a 'fresh kill' effect, when he was shot in the face by Scott Speedman. The second was for the scene showing him one hour later.
According to director Bryan Bertino, the film is partially based on an incident he experienced as a child. One evening a stranger came to his door, asked for someone who was not there, and left. Later, Bertino found out that other homes in his neighborhood had been broken into that night.
During filming, in order to get an actual reaction from Liv Tyler, director Bryan Bertino would tell her where to expect a loud bang from, but would then have the loud noise come from a completely different direction.
Many theaters across the United States were sent faulty reels of the movie, containing sound problems, which made a few minutes to several scenes of the movie filled with nothing but static. Most audiences did not realize the sound was a problem, since the dark overtone and loud background music in some scenes make the static seem part of the movie.
Arguably based on the 1981 Keddie Resort murders in northern California, although this has not been substantiated by anyone connected with the movie, and the writer claiming it is based on a childhood experience.
The trivia items below may give away important plot points.
Before filming any scene after the strangers begin terrorizing the couple, Liv Tyler would have to run laps, do jumping jacks, and other physical activities to get her out of breath, to help simulate the panic her character would have been experiencing.
According to director Bryan Bertino and Liv Tyler, the finale in the original script had much more interaction and dialogue between the victims and the strangers. This was cut in order to keep the intruders mysterious and eerie.
The houses shown during the opening credits each resemble a famous house from various classic horror movies, such as A Nightmare on Elm Street (1984), Halloween (1978) and The Amittyville Horror (1979).