Does Anyone Have Any Tips For Trying To Keep Characters From Being OOC When Writing Fan Fiction? I Seem

Does anyone have any tips for trying to keep Characters from being OOC when writing Fan Fiction? I seem to struggle with knowing what characters that aren't created by me will do and say.

Tumblr is recommending blogs for me to check out and I think they explain my personality

I get to meet a pretty girl soon and I can already feel the panic of picking out an outfit 4 days early

Friend 1: John Quincy Adams!

Friend 2: The apple guy?

My math teacher did NOT just tell me she read the Harry Potter FanFiction "Manacled"