At Neil Marshall's insistence, all the people playing the villainous "crawlers" were professional actors rather than stunt men or dancers. He wanted them to cultivate a distinct character for their crawler, although, in the finished film, many crawlers only appear for a few seconds.
The appearance of the creatures was kept secret from the cast members until the first scene in which they encounter them was filmed. When the cast were finally filming the scene where the girls encounter the crawlers, the girls were genuinely scared and screamed the building down, running off set and laughing.
According to the audio commentary on the special edition DVD, there was a exact replica made of Nora-Jane Noone and after the movie was finished, she got to keep the head of the doll and put it in her mother's freezer as a joke.
Sarah hears the sound of an eagle while she is lying in bed. She goes to the window and it is still nighttime and dark outside. Eagles don't fly at night.
Cars in North Carolina are not required to have front license plates. Also, the license tag is made up of four digits followed by three letters; on North Carolina plates, letters precede numbers.
Why does the birthday cake have five candles in the first shot and six in the next?
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Some viewers have speculated that this is a reference to the lives of all six girls being snuffed out. Another theory is that it was a "happy accident." Her daughter was five when she died in the car accident and since the majority of the film takes place a year later, it could be assumed her daughter would have been six years old and Sarah is celebrating that 6th birthday with her daughter (even though it was a vision and not reality). However, on the DVD commentary, Neil Marshall and his editor acknowledge that this was simply a continuity error.
Q
A Note Regarding Spoilers
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The following FAQ entries may contain spoilers. Only the biggest ones (if any) will be covered with spoiler tags. Spoiler tags have been used sparingly in order to make the page more readable.
Q
What is this about an original British ending?
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The movie had a longer, very different ending on its original release in the UK (unrated version), which was trimmed for the US release (R-rated version). A detailed comparison between the R-Rated version and the Unrated version can be found here.