*nods* For the more serious stories, the writer is usually someone from the same world as that fandom -- the Schuldig Contingent is writing a very involved story about what happened to Schwarz "after the show ended", for example. Zechs is our Gundam Wing writer, Gojyo writes Saiyuki, and Loz and Weiss write FFVII. Occasionally someone will be moved to write in a different fandom, but those are really the exceptions to the rule here.
Crackfics, however... We have the Crackmeisters -- Turk (a Schuldig) and Zechs, and they write some amazingly weird stuff. They don't stay with "their own fandoms" for this: comedy and pwnage are across the board with them. And they have recently taken on an apprentice: Loz is the warped mastermind behind the whole Martha Stewart debacle we did for Christmas.