Consistency

This a Useful Notes page.

There are, roughly speaking, three kinds of consistency that a viewer expects from a story:

External Consistency: Consistency with the real world.

The fictional universe is Like Reality Unless Noted. Violations of external consistency are "unrealistic."

Genre Consistency: Consistency with other fictional works.

The fictional universe should behave like other works in its genre, unless specifically noted otherwise. Any fictional concepts, characters, or settings borrowed from other works should behave as they do in those works. For example, a dragon is generally expected to be a flying reptilian creature that breathes fire. If your work takes place in an Expanded Universe, you're generally expected to be consistent with Canon.

Internal Consistency: Consistency with itself.

Any rules, events, settings, or characters that have been established within the fictional work continue to exist and function as they did previously, unless otherwise indicated.

Consistency aids Willing Suspension of Disbelief, while violations of consistency may be jolting and unexpected. The viewer would be quite surprised to learn that in your universe, Hitler was a circus performer, dragons are scared of fire, and that those two characters who previously knew each other no longer recognize each other. Generally, if a work is inconsistent, the viewer expects there to be a good reason for it. On the other hand, sometimes violations of consistency go unnoticed even if they're quite obvious, or may even be expected; e.g. The Coconut Effect violates External Consistency.

Often, a feature in a work is consistent at one level and not at another; for example, maybe your vampires glitter, which is not genre consistent with other works featuring vampires, but as long as they always do that, it is internally consistent. If a work forgoes External Consistency in favor of Genre Consistency, you have The Coconut Effect. If conversely a work forgoes Genre Consistency in favor of External Consistency, then you have Reality Ensues.

Sometimes, as in the case of sequels, it can be unclear whether two works are distinct works or part of the same work, making the distinction between Genre Consistency and Internal Consistency a bit fuzzy.

An Omnipresent Trope, since every work of fiction has some kind of consistency (maybe except Finnegans Wake).

No examples please—this is just a descriptive Super-Trope and index.