Cristina Raines, under contract with Universal at the time, almost didn't get the leading role due to a female executive at Universal who was more interested in pushing other clients for parts and ignoring her. Raines was told this directly by Michael Winner, who went against the executive's wishes and cast her anyway. Cristina later said this may not have been for the best, however, and as she didn't like Winner's maddening approach to directing. The actress says she cried every day on her way to work, and admitted in a recent interview that she has never watched this film due to the treatment she received on the set.
Cristina Raines has said that the film crew actually used the exterior and the interiors of the Brooklyn Heights brownstone for the film. Raines says a lot of strange things happened while they were shooting there, particularly when she was assigned an apartment to use as her dressing room and found out it was being rented by a priest.
Academy Award-winner John Williams was originally supposed to score the music. However, when Williams backed out to do a project by George Lucas, Michael Small was later supposed to be a replacement. Small eventually was unable to do the score which led to Golden Globe-nominee Gil Melle, who composed a chilling score which blends the elements of Williams, Jerry Fielding, and James Horner.
The book has a sequel entitled The Guardian. In an ironic coincidence, the year 2006 saw the release of movies entitled The Sentinel (2006) and The Guardian (2006) which are entirely unrelated and having nothing to do with New York or supernatural activity.
The trivia item below may give away important plot points.
There was a bit of controversy surrounding this film upon its release when it was leaked that director Michael Winner used real human oddities for the Dead rising up from hell scene at the film's conclusion.