The role of Alex Farnham was originally to be played by George Hamilton.
Nigel McKeand stated that the creatures seen are more 'lumbering' than he intended in the script; his version of the creatures were quick, thin, and more demonic.
The idea of the creatures coming out of an ash clean-out pit came from the Spanish house that Nigel McKeand was living in at the time; it had an old fireplace at the rear with a deep clean-out pit, bolted at the rear. It was so deep, creepy and dark nobody ever wanted the job of cleaning it out.
The total time of filming, including script approval by Lorimar, casting, special effects, voice-over and exterior shots were all completed in slightly over two weeks, one of the quickest made-for-television movies ever made; this was due mainly to a writers strike that had begun just before the script was completed.