On the DVD commentary Patrick Wilson recalled that while filming on the roof, he had to shoot a scene where he yells, "You're not gonna shoot me," five times. After the third or fourth take someone within earshot - not part of the film crew - called the police thinking an actual attack was occurring.
The screenplay's inspiration came from Japan. Producer David Higgins had read reports of Japanese schoolgirls ambushing men who surfed the Internet for underage dates and later developed the story.
Some working titles were "Vendetta" and "Snip Snip." "Hard Candy" was finally chosen because it implies both sweetness and spice--and because it was slang for an underage girl among Internet pedophiles.
The way Jeff gets the gun off of the middle of the bed even though he is tied to an office chair was completely Wilson's idea. Instead of coming up with some choreographed maneuver, he told David Slade to just put the gun on the bed and he would figure it out. The results are more than a little impressive.
Filmed in 18 days.