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.
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.
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.
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.
The trivia items below may give away important plot points.
Camilla Belle was told to hit Tommy Flanagan during the scene in the atrium. She missed him and punched the bridge instead, cutting her knuckle and scarring her hand. She also really did slam her head into a glass window during the fight on the stairs.
The revelation that the calls are actually coming from inside the house was meant to be a plot twist, despite the fact that the trailer, marketing, and even the back cover of the DVD and Blu-ray completely gives it away.