Genre boundaries

A couple of posts back, I said I preferred writers keep rules of magic or super-science as simple as possible; the same is true of genre classifications in movie books.
It’s essential to draw genre boundaries so that you have some idea what movies should be included, and to draw them clearly enough that readers have an idea what to expect—e.g., does your definition of horror cover strictly supernatural stuff or mundane serial killers as well? Does “ghost story” include stories where the ghost turns out to be a Scooby-Doo type phony?
Admittedly, this doesn’t always work: I’d have thought Cyborgs, Santa Claus and Satan‘s “Made for TV Science Fiction, Fantasy and Horror Films” subtitle made it clear what it was about, and at least suggests my horror interest leans to the supernatural side. Nevertheless, a couple of reviewers and some buyers bitched that they’d picked it up anticipating TV serial-killer movies, and were stunned to discover I covered SF and fantasy, which didn’t interest them.
A good example of how not to do it was a book I read recently on disaster movies. It specifically defines the genre upfront as the kind of big-budget, big-star vs. force of nature film I watched back in the seventies (Towering Inferno, Poseidon Adventure, Earthquake) but then it throws in films such as Fearless, which does have an airplane crash but is a serious character-centered drama, nuclear-war films and giant-insect SF films. While these arguably have some disaster-movie characteristics, they don’t fit the definition given; they don’t fit most people’s definition of disaster films; and having included them, the authors leave out quite a few (if you’re going to claim them for your own, don’t cherry pick).
On the other hand, endless parsing of genres doesn’t work for me either. I read a magazine article years ago on SF films that seemed to spend a quarter of its length explaining that Demon Seed was NOT an SF film, but merely horror. I’ll agree it was a horror film, but arguably a computer genetically engineering a human body to hold its AI is SF as well. I’d sooner have a movie include debatable genre members than exclude them (I devoted a lengthy appendix of my book to borderline SF and supernatural films)—although as my reaction to the disaster-movies book points out, that’s not an excuse to throw in everything but the kitchen sink.
For The Enemy Within, it was tricky at first because there’s no recognized genre of “fifth column” or “political paranoia” films, and while there’s a boundary between paranoia and realistic spy films (Breach, with its portrayal of real-life spy Robert Hansson, is on the realistic side, for instance), it’s hard to say where it falls. On the other hand, including every film where one of the good guys turns out to be a spy would be ridiculous. So I state my general standards at the start (one spy=plausible, hundreds of spies across America, much less so) and hope the readers will go along with me.