 The Hill House in the film was not a set, but an actual house.  Although the exteriors were shot at a real house on location, all  the interiors were carefully designed sets on sound stages. While  shooting exterior night scenes on location at the real house, Russ  Tamblyn has shared a story of having chosen to take a stroll through  a cemetery at the rear of the property and having had an experience  nearly as terrifying as the film itself. You can hear his story on  the commentary track included on the DVD of the film.