The car that Harry Andrews drives in the film is a Pontiac Parisienne, formerly owned by Pink Floyd's Syd Barrett and re-sprayed pink for the film. You can see Pontiac on the rear of the car when Sloane drunkenly returns to the house one night and Parisienne on the boot when Andrews removes a shotgun from it.
The original Broadway production of "Entertaining Mr. Sloane" by Joe Orton opened at the Lyceum Theater in New York on October 12, 1965 and ran for 13 performances. The Joe Orton play was the basis for the screenplay by Clive Exton for the movie version.
When the crew needed dressing rooms, they asked at Marmora Road which is opposite the cemetery where the house in the film is. Beryl Reid refused to "lower" herself to using an ordinary house as her dressing room and they had to get a caravan for her and park it in the street outside.
The bikini-clad blonde who appears in some posters for the film is the girl at the swimming pool who exchanges looks with Sloane while Ed is rubbing tanning oil on his back. She has no dialogue, appears only briefly, and is uncredited. Her presence in the poster seems to be an attempt to give a misleading impression about the story (Sloane has exploitative sexual relationships with a middle-aged woman and her brother).