Screenwriter 'Carpenter, John' 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.
Donald Pleasence's character, Father Loomis, is named after the character he played in the Halloween movies.
'Jameson Parker' , who plays Brian, was sporting an old leg injury he picked up while making TV series "Simon & Simon" (1981), and would sometimes need to rest in his trailer because of the pain.
When actor Peter Jason, who plays Dr. Leahy, introduces himself to Dr. Birack ('Victor Wong' ) for the first time, he wasn't expecting Wong to walk away in the middle of the scene. Jason's subsequent confused appearance and awkwardness is natural and unscripted.
Rock singer Alice Cooper is among the hordes of homeless people that surround the church during the film.
Peter Jason hurt his shoulder in the scene where he and Jameson Parker try to break the front doors of the church down to escape. He claims that the pain still flares up to this day.
The movie was shot in just over 30 days.
The scene where Wyndham (Robert Grasmere) is stabbed by the bag-lady at the back of the church is inspired by a smiliar stabbing scene in the Terence Fisher movie Curse of the Werewolf.
Alice Cooper used the bike impalement trick in his stage shows prior to this movie. The bike he uses was his own personal prop.