When writing a fan-fiction, how much do you allow yourself to assume the readers already know?
For example, let's say you are writing a pokemon story (shocking, right?). Do you feel the need to describe the poke-universe (the concept of battling; the use of pokeballs; the fact that there are types; etc. Just the basics of the whole world), or do you assume the reader knows that? When introducing every new pokemon, do you go to great lengths to describe them, or so you assume your reader already knows what, say, a Gyarados looks like if it isn't an important chracter nor its appearance relevant to the story?
Do you think it is a better idea to write your story as if introducing the world to a stranger, or do you think it's better to forgo description your readers will certainly already know and get on with the tale?