Often times, there's a bit where I go "Oh, WTF?" in canon. Or, I realize that I'm going to have to answer that question without ever actually getting the official one. And, occasionally, because I have ideas that crash together and refuse to go away.

More often than not, though, you'll see the headcanons and AUs start appearing because the secondary characters are off-screen enough that you can start speculating about what they're doing and what they're interested in. And that causes all sorts of fun.