When I was taking a film studies course from George Toles he at one time polled the class to discuss what defines science fiction as science fiction instead of fantasy, because he said he didn’t really get it. I think at the time I stumbled through a variation on Tom Shippey’s “fabril literature” definition, even though I did not fully understand it. (And I would now think that definition isn’t very useful.)
You can see my more current understanding in one of my earlier posts, where it’s driven by extra-narrative curiosity.
I’ve also since come to understand his perspective better, in that when you could watch an absurd surrealist movie which follows dream-logic, or you could watch neo-realist films or documentaries, this hair-splitting about which impossible things are permitted in science fiction vs which are permitted in fantasy could look kind of silly.
Sometimes fantasy vs. science-fiction feels more like which family of conventions it will borrow from. Dragons? Fantasy. Laser guns? Science fiction.
You have something like Eternal Sunshine of the Spotless Mind where they have a machine that alters your memory, but because it isn’t set in the future and there’s no aliens or laser guns, most people don’t think of it as science fiction. Though that’s largely how the genre of film science fiction has played out. The same story would be perfectly at home in written science fiction.
I also finally watched Phantasm recently. The way it included science-fiction elements in the horror genre made me think of Ray Bradbury or Stephen King, which shows how rare it is in film. And yes, horror tends to be seen as trumping the science fiction as a genre, in the sense that something like The Thing can be about an alien that emerged from a flying saucer but it’s normally thought of as a horror film, not a science fiction film. Alien will be more likely to get counted as a science fiction, because it’s In Space.
Even things like King Kong or Godzilla will often be thought of in terms of science fiction or fantasy. Is it that setting trumps all else?
I prefer to try to dig down and find something deeper than setting. Like how people can argue over whether Dirty Harry is an urban western, or that Star Wars is more fantasy than science fiction, I try to find out what is the narrative feel of science-fiction.