When Dr. Leahy, introduces himself to Dr. Birack for the first time, Peter Jason wasn't expecting Victor Wong to walk away in the middle of the scene. Jason's subsequent confused appearance and awkwardness is natural and unscripted.
John Carpenter states he set out to make a film that was atmospheric and dreadful. At the time he was noticing a lot of derivative horror films and wanted to try something new involving quantum mechanics and religion.
The audio of the "broadcasts" that Brian receives in his dreams was sampled by Marilyn Manson in the song "Down In The Park" originally performed by Gary Numan. It is also sampled on DJ Shadow's album 'Endtroducing...'
When Brian and Catherine are in bed in the morning, Brian says to her, "Who was he, the one that gave you such a high opinion of men?" This is a fairly well known quote (among others) from To Have and Have Not (1944), only the sexes are reversed here. The original has Lauren Bacall saying, "Who was the girl, Steve?" To which Humphrey Bogart replies, "Who was what girl?" And she responds, "The one who left you with such a high opinion of women."
Screenwriter John Carpenter is credited as "Martin Quatermass". The pseudonym is a homage to Professor Bernard Quatermass, the lead character of The Quatermass Experiment (1953) and several subsequent TV series and film versions. In the original press notes he was described as 'the brother of Professor Bernard Quatermass, head of the British rocket programme.' In a further nod to Carpenter's influences, the character Wyndham is named after science-fiction author John Wyndham and Anne Howard's character is named after the star of The Wasp Woman (1959), Susan Cabot.
Alice Cooper originally asked John Carpenter if he could come to set just to watch a special effect, due to Cooper's manager also producing the film. Carpenter eventually decided to offer Cooper a role as the leader of the street people.
The genesis of the project came from Debra Hill describing a dream she had of a vague dark figure exiting a church which filled her with dread. John Carpenter developed the story around this idea in hopes of recreating the fear Hill really felt.
Although the film wasn't John Carpenter's first screenplay that he wrote since Escape from New York (1981), it was the first film since Escape that he directed but credited himself as "Martin Quatermass". A similar thing he did on They Live (1988) but under the name "Frank Armitage".
When Etchinson is cornered in the alley by the homeless people, he is listening to "Prince of Darkness" by Alice Cooper on his headphones, just as he is about to be killed by the character played by Cooper.
The trivia item below may give away important plot points.
Kelly is selected by Satan (green fluid into the vase) to be possessed by him, in order to free his father Lucifer, trapped in the anti-matter universe at the other side of the mirrors. Since then Kelly is a woman and therefore capable to bring life, Kelly is turned in a satanic version of Mary, bringing life Lucifer instead Jesus because is selected by Satan instead God.