Tough subject. Not the least because what is, and isn't, canon isn't always that easy for two or more people to agree on. (There's another thing, called "fanon" that I digress on at the bottom of this page.)

Some characters have a slightly different personality every time we see them. Some shows deliberately re-invent some characters in any way this week's story line seems to require.

For the sake of argument, let's say that canon is "exactly" the action and dialogue we see and hear on-screen. If a character's motivation for an action is unexplained--then there is no canon motivation. There can be one or more very plausible motivations--but none of these are canon.

If Duncan doesn't have his sword, then in a later scene he does, it isn't canon that he went home and got it unless he says that's what he did. It's plausible, but not canon. It's also plausible that Joe or Richie or Amanda brought it to him. (Side note: If a character is known to be untruthful, then his/her dialogue that something happened may or may not be canon. It's open to interpretation. If you see them do something, it's canon. In the case of a character as compulsively Honest and Up-Standing as MacLeod, if he says he did something, then it's canon. But there's wriggle room...if you can think of a good reason why he'd lie about it.)

So, what does that leave us with? A lot of half-explained stuff on-screen, surrounded by a universe of gray, non-canon speculations. Slash (really, fanfic in general) is by definition almost entirely non-canon.

Of course, there's non-canon, and then there's NON-canon. Mulder's chronic insomnia in fanfic is non-canon. Based upon scenes in a couple of episodes where Mulder sleeps badly, or not at all when he's under enormous stress, some authors choose to portray him as a man who almost never sleeps. It's possible - we've certainly seen him having trouble sleeping at least some of the time...but it's not canon, you understand. (If the man really never slept, he'd be as psychotic as some authors seem to think he is - but that's a different rant....)

Mulder portrayed as a flamboyant, cross-dressing drag queen who spends his nights doing Scarlet O'Hara impersonations at the local gay bar is NON-canon. We never see any sign on-screen of this personality. It's certainly the kind of thing his co-workers would exploit and they seem to be aware of his every other foible. His on-screen humor, while occasionally bizarre, is never flamboyant. This is based on nothing we see on-screen.

Sure, you can choose to portray him this way. I'm not denying anyone the right to their own choices. I'm just saying that we all leave canon, more or less. Be sure you know how far you're departing from canon and make your choices carefully. They can make or break a story.

FANON

There's canon, and then there's "fanon." If an author, or sometimes a group of authors make an interpretation of some canon scene in one certain way that other fans decide they like, that interpretation of that scene can become fanon. They might even decide that something that's not even hinted at on-screen makes sense for a character or characters, and adopt that idea en masse.

Fanon is a non-canon fact or event that is generally accepted by a relatively large body of that show's fans.

Examples of fanon might include the persistent rumor of Blair's pierced nipple in Sentinel. It includes Agent Mulder on the X-Files having a wardrobe full of Armani suits. (He doesn't. He has some nicely tailored suits, but they aren't Armani.)

Many fans, authors and readers, accept these things as fact, even though they aren't supported by anything we see on-screen.

Understand that it takes more than one, or a handful, of authors or fans screaming their opinion about something loudly to make it fanon.

For instance, (PERSONAL RANT APPROACHING!!) in the X-Files, we didn't see Mulder filing the serial numbers off of Skinner's gun in Zero Sum. While there is a group of fans who insist that Mulder had to be the one that did it, this hasn't passed into fanon.

I, for one, disagree completely. Some fans believe Mulder took Skinner's gun and spent the rest of the night filing off the serial numbers, before he took the gun in for a ballistics test. (If they'd ever tried to file the engraved serial numbers off of modern carbon steel, these fans would know that this is not as simple as it sounds.)

Not only do I feel that such an action would be completely out of character for both men, I've been known to point out that Mulder is wearing exactly the same suit and tie in both the scene in Skinner's apartment and in the Ballistics testing department. (They don't seem to have an answer for this. Perhaps they think he sat around in the suit all night.) I think it makes more sense that the two men went directly from Skinner's apartment to the Hoover Building with the gun. The deserted nature of the ballistics testing department, which is very unlike the bustling, busy area we see in other episodes, seems to indicate that it is, indeed, still the middle of the night.

The point is, there is neither a canon, nor a fanon, explanation of who filed off the serial numbers. I say it was the Consortium because the only reason they stole Skinner's gun and used it in the murder was because they wanted him to know how easily they could bring him down if they decided it would be in their best interests. Other fans say Mulder did it because he wanted to make sure Skinner wouldn't be arrested if the gun did turn out to be the murder weapon. For all we know, Skinner did it himself ten years ago after he bought the gun, you know? Why would he? I don't know...it's not a part of canon and we'll never know. (PERSONAL RANT ENDS)

I brought all this up mostly because I want you to be aware that you'll run into these assumptions, whether they're fanon, or just some people's personal beliefs, in any fandom. Don't let these things restrict your ideas, or your writing.

I said above that we all depart from canon. Fanon is even less restrictive. Of course, you may get questions from the fans about why you forgot Blair's nipple ring when you wrote that shirtless scene, but you'll get fewer questions than complaints than you will when you depart canon and write Blair with a crew cut.