In his DVD commentary, director Dan Curtis explains that he dropped an entire 15-minute opening with the family in New York, seeing the ad about the house in the paper and Ben getting a parking ticket, because it was "boring".
The shot where the vines seize Oliver Reed's legs during the escape attempt were actually done in reverse. The vines were originally unwrapped from the legs and dragged off, but in reverse the vines appear to grab his legs.
The haunting flashbacks that Ben Rolf has about the creepy chauffeur were based off of an actual childhood experience of Dan Curtis. Curtis recalled as a young child being at his mothers funeral and seeing a chauffeur laughing outside of the funeral parlor which disturbed him deeply. He wrote the memory into the film and the chauffeur became a most eerie character.
In the commentary with director Dan Curtis, he says that he actually read the novel 'Burnt Offerings' years before he got the offer to direct this film. Curtis said that he hated the open-ended conclusion of the novel and at the time thought to himself that he would pity anyone who would attempt to make it into a movie. Ironically years later the offer to direct the movie version of the novel came to him and the first thing he did was write an ending that he liked - the one featured in the film.
The trivia items below may give away important plot points.
While shooting the film's finale, where Ben flies out the attic window and falls, the films stunt man had a near-disastrous accident. He projected himself too far out from the window and the crew waiting below had seconds to move the safety padding for his landing. The stunt man almost missed the pad, landing a mere foot or so from the end of the safety pad.