What I loooooooove about fanfiction is that it turns story writing inside out. You don't have to introduce the characters. We know them! You have to reinforce what the readers already know and recognise. You don't have to lay out the setting. We've been there! You have describe the places we know in ways that can feel like going home.
But wait, if it's an AU then you need to set up the setting and lay out the layout whilst reinforcing the characters we already know, making sure they didn't mutate in any way in the drag-and-drop-into-a-different-world portion of your plot-creating in ways that disconnect the reader from who they know and love/hate.
AAAAAAND, the characters have to stay recognisable and consistent throughout every world you plunge them into, every heartbreak you shove onto them, every happy expression (however the character shows that) you snapshot, every drop of blood spilled, every held breath, every wrenched gut, every slamming orgasm, every hand too weak to hold, every plea, every song, every sleepless night, every glorious or interupting morning, every new experience, every old story, every secret whispered, every boundary pushed, every lip quiver, every shake from silent rage.
Through all of that, every reader needs to be able to open up the scene and say, "hello, I know you".
