Though the film takes place in Mexico during the summer, it was shot primarily in Australia during the winter. Heavy coats were provided for the actors in between takes, and water and olive oil had to be sprayed onto their bodies to simulate sweat.
In the novel, there are no "ruins" to speak of, just a hill overgrown with the vines and the shaft cut into it. Director Carter Smith suggested changing it to an ancient Mayan temple, an idea with which writer Scott B. Smith agreed.
Director Carter Smith had just purchased a copy of the Scott B. Smith novel and started reading it when he got the phone call offering him the film adaptation.
The film rights were purchased before the novel was published.
Before filming, Laura Ramsey talked to nurses who had treated people in psychiatric wards to get an understanding of some of the little mannerisms they said people would have if they thought something was overtaking their body and felt things moving around inside them.