Many real aspects of Columbine shooters Eric Harris and Dylan Klebold were used in the film. Most obviously being that the entire film was shot exclusively as a video diary (excluding the massacre itself), as Harris and Dylan did film many videos (later known as "The Basement Tapes") documenting their plan. As of October 2014, "The Basement Tapes" still have not been made available to the public.
During the scene where Calvin and Andre go shooting with Chris, Andre says "natrual selection" while Calvin is firing. "Natural selection" was Eric Harris's (Colombine shooter) favourite law. basically, it states that nature gets rid of the weak organisms, while supplying the stronger organisms.
Like Andre, real life Columbine Massacre assailant Eric Harris worked in a pizza parlor, as did other Columbine assailant Dylan Klebold. And like Calvin, Dylan Klebold attended the school prom just prior to the massacre.
The party scene was filmed in Albany, New York during an actual party with many guests unaware of the filming. The location was the actual apartment of the director's brother, who portrays Chris in the film.