...this is a bad movie. Genuinely bad. Copycat script, awful sound dubbing, cheesy acting, terrible production values; technically, it's an abortion. But despite all this, what stands out, are the visuals; the visual impact of "Beyond The Door" is genuinely terrifying.
To write, produce, and direct a horror movie about demonic possession which is truly original, is nearly impossible. It has been done many, many, many times, by many, many, many people over the years, so it's a given - there will be some thematic material which will overlap with prior work by others, but in this case, some of the scenes depicted in this film from 1974 directly echo "The Exorcist," from 1973: the floating body, the gutteral voice of the possessed, projectile vomiting bile on a person who is trying to help, the rotating head, the list goes on. There was so much mimicry that Warner Brothers, who owned Bill Friedkin's masterpiece, actually took the film makers of "Beyond The Door" to court for copyright infringement. The legal matters worked out, "Beyond The Door" went into general release, and despite being derided and maligned by critics and many ticket buyers alike, it has survived as a cult film (no pun intended).
I have to say, when I first saw it on a weekly Saturday late-night program ("Chiller Theater," for those who lived near Pittsburgh, Pa in the late 1970's), it scared the living daylights out of me. I expected something creepy, but I had no idea. Yes, it's a slightly altered "Rosemary's Baby" with visuals and a soundtrack inspired by "The Exorcist," but lost on those who didn't give it a chance is, this really is a genuinely frightening film. The look on Juliet Mills' face during some of sequences depicting demonic activity stayed with me long (!) after the movie ended. I never expected anything like this; I remembered her from a television series of the 1960's called "Nanny And The Professor," where she portrayed a sophisticated, yes, you guessed it, nanny who was hired to care for a professor's family; a harmless family-friendly weekly situation comedy. So it was shocking to see her acting so strange in this film.
If you watch this movie, just keep this warning in mind: it has many copy-cat elements and it has poor production values which could impede your enjoyment of it, provided you do enjoy something like this.
Objective fact: "Beyond The Door" is very derivitave, and cheaply made.
Subjective opinion: Despite its shortcomings, it is a terrifying film.
Good luck sleeping after watching it.