early introduction to the rules of the world.

Not really sure this is the best place for this question, but anyway...

My girlfriend and I have been watching a show which starts out like completely realistic historical fiction, has two crazy paranormal ghost/demon Halloween episodes, which kill off a major character, and then goes back to being completely realistic historical fiction, like the Halloween episodes never happened (except that the character stays dead).

I feel like this is poor writing. There\'s a Hal Clement quote that explains it well:

\"Writing a science fiction story is fun, not work. ... the fun ... lies in treating the whole thing as a game. ... [T]he rules must be quite simple. They are; for the reader of a science-fiction story, they consist of finding as many as possible of the author\'s statements or implications which conflict with the facts as science currently understands them. For the author, the rule is to make as few such slips as he possibly can ... Certain exceptions are made [e.g., to allow travel faster than the speed of light], but fair play demands that all such matters be mentioned as early as possible in the story ...\"

I feel like that last sentence applies to all fiction. It\'s a rule similar to Chekhov\'s Gun, or Deus Ex Machina: we should be told what kind of story it is early in the story, and if the kind of story changes, it should be a gradual change. For example, if zombies suddenly appeared in the last page of Pride and Prejudice, and ate Mr. Darcy, that would be bad writing because Pride and Prejudice is obviously not a zombie story — however Pride and Prejudice and Zombies works because the reader is told early in the story that this is a zombie story.

Am I completely off base with this?

Probably the right place, since it's about tropes like Willing Suspension of Disbelief and what's on Ind ex Machina.

I wouldn't say "as early as possible", but they should be mentioned or hinted at sufficiently early. How much earlier is necessary depends on the magnitude of the change, since smaller stuff can (usually) get away with less time without breaking Suspension of Disbelief.

Another factor is what role the revelation has. If it's not something that solves a problem (or creates one not easily solved) you can perhaps get away with introducing it right there and then, in which case that scene works as the introduction of it.

Your example sounds like a Big-Lipped Alligator Moment, as it's not referenced afterwards.

How to Stop the Deus ex Machina is an interesting page on the topic.