The first 30 minutes of the film has a background noise with a frequency of 28Hz (low frequency, almost inaudible), similar to the noise produced by an earthquake. In humans, it causes nausea, sickness and vertigo. It was a cause of people walking out of the theaters during the first part of the film in places like Cannes and San Sebastian. In fact, it was added with the purpose of getting this reaction.
During the party scene shortly after (or before) the rape, when Vincent Cassel is asked his name he replies "Vincent" instead of his character's name, Marcus. He quickly covers this up by saying "just kidding" so that they would not have to reset the long and complicated shot.
The "Rectum" is in fact a genuine gay S&M club in Paris, Club Banque, which means Club Bank since it is located in a former bank. The crew changed the name, redressed the set and added red lighting. The club was spread across the basements of three separate buildings and was so cavernous and confusing that many of the crew members became claustrophobic in it.
This began life as a study of a committed relationship for director Gaspar Noé. He wanted to make the film with a real-life couple and husband and wife Vincent Cassel and Monica Bellucci ideally fit the bill. However, as the trio started workshopping ideas for the film, it started to take on a much darker hue.
Monica Bellucci and Jo Prestia completed six takes of the rape scene over two nights. Bellucci claimed the first take was actually the easiest to do, since on subsequent takes she had a heightened awareness of what was going to happen and had to prevent this from interfering with her characterization.
In "An Experiment with Time", which Alex is reading during the last (i.e. chronologically first) sequence in the film, J.W. Dunne postulates the existence of a "time-travelling observer", which in dreams can move backwards or forwards in time to actually observe events which may not have yet happened. These are the 'premonitory dreams' which Alex mentions to Marcus and Pierre. Alex earlier describes such a dream to Marcus, where she is in a 'red tunnel' which breaks in two.
Anywhere from five to fifteen takes of each ten to twenty minute sequence were made, some as short as three minutes and some as long as fifteen. Some takes were assembled using invisible edits disguised with digital post-processing.
The entire film was shot on Super 16, telecined to high-def video for color tweaking and editing, and then exported to Super 35. For many of the handheld shots, the director used the smallest existing 16mm camera, the Minima.
The trivia item below may give away important plot points.
During the rape scene a man in the background walks in the tunnel and then walks out. This was a accident as an assistant on the film accidentally walked in and walked out when he realized they we're shooting. The director kept this in because he wanted to show that not everyone prevents crimes