There's a great Ursala Le Guin essay that's (re)printed in The Secret History of Fantasy (primarily a short story collection with a couple of essays). In her essay, Le Guin points out that while fantasy fiction and surrealism differentiate themselves from realism by dealing in that which is realistically impossible, fantasy and surrealism are not interchangeable.
Surrealism, she says, is when the rules of reality are subverted.
Fantasy is when the rules of reality are subverted, and replaced with a new set of rules.
Therefore fantasy is more like realism than surrealism. You read fantasy fiction and realism expecting a certain set of rules to shape and guide the world and your reading of it. Therefore, if something strange happens in a fantasy story, you don't ask if it was a dream. Fantasy authors aren't subverting rules, or trying to trick your mind; if they tell you something happened, then it happened. Believe the words on the page: it was real, not a dream.
(You have no idea how badly I wish I could have gotten my MFA workshop groups to read this Le Guin essay or at least listen to the above statements, but I digress.)
And so the fantasy reader enters into the novel having a contract with the finite world. The realism reader makes that contract with the author before she opens the book. The fantasy reader gives the author (roughlyt) the first 50-100 pages to set the terms of the contract. She'll believe events that are based off of that contract -- but throw any "extras" at her, and she's likely not buying.
I've been struck more than once by a situation (often near the end) which a dramatist would call deus ex machina -- essentially, a moment when a god or some other device, enters into the novel and "fixes" the narrative problem, allowing everyone a happily ever after.
Why am I writing about this specifically in the context of fantasy? Because sometimes you'll be reading a fantasy novel with, say, vampires and then the narrative problem will be solved by the appearance of, oh let's say, a ghost.
It's all gravy, right? I mean it's a fantasy novel -- we expect fantastic things, right?
The appearance of the ghost is a form of deus ex machina because the sudden appearance of the ghost broke the contract with the finite world. In Le Guin's terms, the fantasy broke the rules of reality that it had established for itself.
If the viability of ghosts in this vampire world was established early on as part of the rule building / world building, then the sudden interruption of a ghost at the end of the story would not warrant a cry of deus ex machina. It would warrant perhaps a subtle oh, another ghost -- clever.
Yesterday I posted about series novels, and discussed the ever-growing superpowers as a breech of contract with the finite world. If we're sticking with the contract metaphor, then every-growing superpowers (the kind that get surprisingly bigger and better with each book) would be a legal gray area. Not an outright violation, but not exactly in the spirit of the contract either. As a reader, I'd much rather see characters deal with the situation using what they have, getting cunning and clever -- after all, making a character work within the rules of her reality makes her story resonate with the human experience, and that's what all fiction, no matter the genre, should strive to do.