I'm referring to all the characters in the show, dead and alive. (*plays salute song to the list of fallen characters who we've never recovered*)

OCs are one thing: they're your own creation, so people can't possibly criticize you for getting your character's personality wrong. 

With canon characters like our beloved humans, angels, demons, and everything else in between...

Yeah. People start apocalypses because our humans, angels, and demons aren't themselves. If the change is minor, people may brush it off. Or, you may get extremely devoted fans who will pull you apart for the small change.

People can be brutal, even more so when they hand out criticism. Words do hurt. But sometimes, you gotta "shake 'em off" *starts blaring Hillywood Supernatural Parody* 

I regret nothing with that. And if you have never heard of that parody, you are freaking. Missing. Out. (But beware, minor season 10 spoilers.)

Okay, me as a fan of the show, I like to see the characters stay true to who they are (unless there is a viable reason for their personality change). Seeing any characters do something that's not them, well, it just gives me an off vibe. Like, "Wait. What? They wouldn't say/do that. Why are they doing that?"

So, some tips on keeping canon characters who they are?

*Observe. Watch a few episodes, get a feel for whichever canon characters you are including in your fanfic. If you're going through a season, read the transcripts carefully. Look at the words, look at the actions. Read up on the characters if you want to not disappoint your readers. When editing, read carefully. Is this something this character would do? If not, maybe it's time to tweak that bit.

*Use your knowledge. No matter what you're writing, an AU or something else, knowledge is power. You don't want to portray Dean as a scaredy cat (unless he's got a case of that ghost sickness again...) if that's not what he is at all. 

You don't want to have people see Cas as the villain if he's not (unless Lucifer's riding his vessel...).

 And you especially don't want Sam to be a heartless dick (unless he's lost his soul again...damn it).

*When in doubt, ask others. Having trouble? Ask a fellow hunter! We are a community, a family. Trust me, it can be fun to bounce ideas off of people, no matter how big or how small. Some (myself included) would be more than happy to help.

Thinker: Has there been any fanfic in which you've seen a canon character act out of character? Was it for good reason?