To spare people who know no more than I did about 'Spider', the helpless voyeur fly on the wall throughout almost every recollected scene in the movie, I'll write nothing else of the character study.
Watching this late at night left me telling myself, "Uggh! That was good, but I don't want to see it again." But after lying in the dark for a while and absorbing it, before my mind maybe went to work while I slept on it, I did.
Now I knew what happens at the end, a second viewing was the only way fully to appreciate the skill Cronenberg exercised in filming a novelist's revision of his work. In an article at 'eye WEEKLY', without spoilers but with insight, Jason Anderson writes that unlike Cronenberg's "controversial adaptations of Crash and Naked Lunch, this movie had a screenwriter (Patrick McGrath, who adapted his 1990 novel) and an actor (Ralph Fiennes) long before it had a director."

It took Anderson's piece (and a sensible word from a Québecois writer, 'man-man-dot-org,' who notes that "the regular IMDB approach of watch-a-movie-write-a-review has done Spider a grave disservice") to remind me to look twice before leaping in.

'Spider' is one of the Canadian director's slowest, darkest and saddest films, much more mature than 'The Dead Zone', which took me by surprise given an unpromising premise.
I would guess, however, that much of the credit for the detailed rightness in Cronenberg's vision of the unhappier parts of the Britain I grew up in — those dreary clothes, the dads with their allotments and sheds, the language and life of the poorer London streets — goes to McGrath and to Fiennes and others in the adult cast.
I'm glad I was in a cheerful mood when 'Spider' went into the DVD player last night, because it's a demanding film, close to exhausting first time round, before it starts to work on you, making a different contribution to your perceptions of "reality" from 'eXistenZ' but one which is also richly worthwhile.