Even though the stranger is played by Scottish-born actor Tommy Flanagan, the voice in the phone-calls is actually Lance Henriksen's. Henriksen fit the voice, whereas Flanagan fit the build of the stranger.
There were at least five different versions of the peach/coral colored t-shirt Jill wears throughout the movie; a special "waterproof wet-look" version was made for her to wear through her scenes with water.
The front of the Mandrakis' house was built at an unused reservoir, while the interior was in a studio.
Though the film is billed as a remake of the 1979 film of the same name, it is actually only a remake of the first 20 minutes of the original film. These first 20 minutes are what made the original a cult classic.
Camilla Belle almost turned down the role of Jill due to her personal dislike for horror films. Simon West convinced her that he was going for a more psychological thriller, so she agreed to do the movie. She had to turn down a role in another horror remake Black Christmas (2006) due to commitments to this film.
The voice of "Stacy", the unseen girl during the opening credit sequence, and victim of the killer that targets Jill, is that of director Simon West's 12 year old daughter Lillie.
In preparation for her role as Jill, Camilla Belle had to do two months of weight training and learning how to run.
Director Simon West opted not to show the crime scene Detective Hines has to see at the beginning of the movie to secure a PG-13 rating.
A poster on the wall in the hospital in the final scene reads "No shame, no blame, no names". The stranger's name is never revealed in the movie.
When Tiffany (Katie Cassidy) first appears in the movie, she's stirring a jar of red paint while standing next to Scarlet - behind them are several jars of red tinted paints. The name for a woman who is promiscuous or loose is is often referred to as "a scarlet woman", a subtle reference to what Tiffany has done to betray Jill.
Screen Gems had greenlit a sequel rumored to be titled "When a Stranger Returns" in which Hayden Panettiere was to play the babysitter. Screen Gems later cancelled the project.
Jill's high school is "Fernhill". Director Simon West went to a "Fernhill High School" in his native Herfordshire, England.
To promote the film, AOL Instant Messenger ran ads beckoning users to IM Jill020306. When messaged, "Jill" (a Colloquis-style program) made small talk before panicking, as she received calls from a stranger asking her to check the children. She then gives the user her phone number (a toll-free 877 number) and asks them to call her. When users call, they hear an ad for the movie. To coincide with the DVD release, a new screen name appeared, Jill051606, to tie in with the DVD release date on May 16, 2006. It does not involve calling her, but instead she directs you to a video security system on the official DVD site where the shadow of the stranger passes by frequently.
Tommy Flanagan and Lance Henriksen previously collaborated on Alien vs. Predator (2004). They play archaeologists in that film, here they play the same character.
Tommy Flanagan sang a Broadway version of his infamous "your blood all over me" line to Camilla Belle in between scenes.
MySpace ran an advertisement: the profile for Jill051606, in which users can add the profile as a friend, leave comments, and read Jill's blog.
Jill's locker at school has a mini-white board on the inside door with the words "That's Hot!" written upon it; she also has Polaroid of Tiffany (Katie Cassidy), who she has been betrayed by.
Tessa Thompson and Clark Gregg both have roles in the Marvel Cinematic Universe with Gregg appearing as Phil Coulson in Iron Man (2008), Iron Man 2 (2010), Thor (2011), The Avengers (2012), and Agents of S.H.I.E.L.D. (2013) while Thompson appears in Thor: Ragnarok (2017) as Valkyrie.