The seedy motel sequences were shot at the Wampum Court Motel in Sylmar, CA, a 1950s relic that had definitely seen better days. To enhance the fleabag image, two gaudily-painted signs were added for the film, touting "air conditioning" and the deliberately misspelled "kitchins."
The part of Morticia was originally offered to Cher, who passed because she felt that "The Addams Family" wouldn't do enough to boost her slumping (at least at the time) film career.
Charles Addams' cartoon characters had no first names until 1963, when he gave them names for the TV series. Pugsley was Pubert until ABC nixed that as "too suggestive." Addams named Gomez for an old family friend, and Wednesday after a line in a nursery rhyme, "Wednesday's child is full of woe."
Thing was played by the hand of actor and magician Christopher Hart. In the TV series, Thing was said to be played by "Itself," but was actually the appendage of Ted "Lurch" Cassidy most of the time.