You have to worry about keeping them in character. I find it somewhat easy to make up OC's. I enjoy making up OC's in fact and I'm usually decent with fleshing them out.

That's true especially when you're not familiar with the fandom but it is hard to do a pairing cause you have to fit the oc character into the fandom plus deal with original fandom ship (tons of fun especially dealing with die hard shippers, ugh!)

I've only had one OC. I have tons of OCs on DeviantArt I've never used any of them on Fanfiction.net.

Do extras that weren't in the movie count as OCs? I've had a few of those.

Sure, I guess they count as original characters, too :)

@ktchenor Oh yeah, I've tried my hand at doing that a few times. I feel like its easier for certain fandoms than others. Like the Avatar fandom I'm usually pretty good with inserting OC's. It's a bit harder with Harry Potter in my opinion. Luckily I haven't had a run in with any die-hard shippers yet.

You have to try to keep canons in character. With an OC, you can pretty much make it all up, though you do need to try to keep their traits consistent.