Contrary to the marketing's claims, Wes Craven had no involvement in the production of the film. The complete U.S. title for They is "Wes Craven Presents: They." Craven was considered an 'executive producer' of the movie. However, other than lending his name to the title, he had no part in the making of They - Incubi dal mondo delle ombre (2002). It's assumed that the purpose for putting his name in the title was to publicize the movie and attract horror fans.
In the original unaltered script by Brendan Hood, the plot dealt with a group of four recent college grads who discover that Earth is actually run by a race of organic machines which allow the human race to exist so that they can kill them and harvest their corpses for 'spare parts' as "Their" bodies break down and deteriorate.
Though Brendan Hood is credited as screenwriter, he only came up with the basic idea. Everything in his original draft was subsequently written out by the producers, and it's estimated that up to 10 people have worked on the script. Only one of them was credited for the revised script.
This film was Radar Pictures' first film production.
Jessica Amlee filmed an alternate opening scene featuring a young Julia sleeping and being terrorized by "They" in her bedroom, in contrast to the theatrical cut which features a young Billy instead. It was cut and reshot after test screenings, and Jessica Amlee only remains in the film via photographs and a short scene featuring VHS footage of a young Julia awakening from her night terrors in a mental hospital.