David Cronenberg received the screenplay from Patrick McGrath out of the blue, with a note attached saying that Ralph Fiennes was interested in playing the part of Spider. After about four pages, Cronenberg had decided that he wanted to do the film.
The trivia item below may give away important plot points.
Spider's difficulty with his family history is a real psychological syndrome called the Capgras delusion. Persons afflicted with the syndrome are convinced that a person close to them has been replaced by an identical-looking impostor.