Every story has them, or at least every GOOD story has them. That one eccentric character with the quirk or odd way of behaving that draws him out to the reader’s eyes. A character that adds flavor and some fun to the telling of the tale. The eccentric character.
First let’s clear something up. Eccentric does NOT mean crazy, although it often could. An eccentric character is a little bit sideways of the norm, something about him that makes him/her/it stand out. It could be the way he dresses, or the way he acts. Or even something he carries. The lady talking to a tree log she carries with her is eccentric, but so is the Physicist that loves to play the bongos (the real life Dr. Feynman loved to play bongos). You start by first realizing what constitutes “normal” for the world in which you write, and then deciding on what quirks would be make someone stand out. But don’t just make them random quirks; everything has to have a reason. The eccentric character must have a developed personality that out of which these apparent quirks would make sense; it is just in the context of the rest of the world where they seem odd.
Keep things consistent. He can have more than one little quirk, but they must all fall out of the main reason for them; his personality, background, and whatnot. Also, an eccentric needn’t always be comedy relief. In fact, it can be an otherwise very serious and important character. It is just that there should be something about him that stands out beyond the rest. This also implies that you should not have too many eccentrics crowded together or they won’t be eccentric anymore. One unusual eccentric per grouping of characters is plenty fine.
It could be something as simple as the guy always has a packet of jelly babies on him because he loves to snack on them. Or maybe he tripped on acid one time to many in the past and burned out a part of his brain. He might dress differently, behave oddly, because he came from an altogether foreign culture where such dress and behavior would be considered normal.
Of course he really COULD be crazy, but that doesn’t mean completely random, and it also doesn’t mean he’s an unimportant character. I have a wizard in my stories that is completely off the deep end crazy, and there are reasons in his past why this is so. A powerful wizard but just being crazy doesn’t mean he’s after world conquest; there are specific things he wants, but within that his moods are unpredictable. He also wears a hood due to his more-than-ugly features, though he isn’t above unhooding himself to drive a couple of other people crazy.
And there is something to be said about how eccentric such a one should be. The bigger the story the more room there is for an extreme eccentric without crowding out the other characters. Make the eccentric stand out, but not take over.