Inspired by a true story.
In the booking photo, Jesperson is shown as being about 6 feet tall. The actor playing Jesperson, David Arquette is 5 feet 10 inches tall. In reality, Jesperson was 6 feet 6 inches tall and weighed 240 pounds.
The happy face killer did not actually mark his victims with a happy face. When he wrote many letters into the media and authorities, he would draw a happy face on them.
Contrary to what was shown, the FBI had minimal involvement in the several homicides. The cases were investigated by the local agencies the FBI supervisor had demeaned early in the movie.
There was a women who turned in her boyfriend and took credit for the killings, but as it turns she was just trying to back at him for being a jerk. The real Keith Jesperson was offended by the fact that they took all of the credit since he was so proud of his killings.