well there's also the difference that a fanfiction is a story without much background because it gets explaned either in a prologue or throughout the course of the fanfiction (atleast that's how I'd write it) whereas the roleplays start with a brief introduction with all you need to know in the OP and from there everyone joins in
This may be true for some, I understand, but what about SoS? That goes with the characters decision. It's like a story with way to many main characters! The main character leads the fanfiction, and the characters lead the RP. That's what I think, at least.