Word of God
is when the work's creator fills in details from this side of the Fourth Wall, Fanon
is when the work's fans infer their own consistent theories on details that haven't been fleshed out (and Jossed
is what happens when the latter gets trumped by the former, or by new developments in the work itself).
is when an actor (or other supporting crew, such as an artist or prop designer) within a work fill in their own details independent of the creators. Sometimes, there just isn't much detail to go on, and they will come up with their own ideas to explain a character's motivations, or how some piece of technology should work, or the reasoning behind a certain style of clothing favored by a character or group of characters.
This can happen for various reasons, such as helping an actor determine the best way to approach their character without having to pester the director with questions such as "What's my motivation?
", particularly if their role is relatively minor.