I say capturing the essence of the character. I have a hard time doing that sometimes, and it's in my opinion what will move the story forward or not. If the character isn't "in character", then what's the point of RPing or writing Fan fiction? I don't see much of a point at all.

Everything else seems to fall into line if you have the essence of the character down, you know?

--
This is the best kind of person to talk to: someone who is willing to take an honest look at the evidence. Being willing is essential. Evidence cannot convince the unwilling.

-- Norman L. Geisler, Frank Turek, David Limbaugh

Theres more to characters than physical appearance and role in the story. Duo is the prime example of that. Think of how much of his past makes him so oober in the story? So essence and backstory and such is important. Well, all of it is important...
Which is probably thoroughly unhelpful.

--
I know, my name should be Spodmistress Spiff, but that makes me sound like an emo hooker. I prefer being 100% lame and slightly Calvin and Hobbes.

"Are we not all, in some way, searching for our cow?"

I voted for essence. I figure if you get that, most of the other things will usually fall in place.

--
The last thing I want to do is hurt you...but it's still on the list.

I voted dialogue and mannerisms, but I'd have to say the mannerisms are more important than the dialogue by far. Making sure to have dialogue is sometimes too much. However, all of these aspects are important.

--
思いにふける物
[la Dolce] C'est le vie

I vote for all, but if I had to choose it'd be to drive the story, which would be their ultimate purpose.

But that probably just comes from playing Zechs. His whole purpose as a character is to drive the overarching storyline of the whole show.

--
Glorious leader of ~THE-AXE-GANG

Proud outcast from the ~Anthro-Alliance

On Twilight: One can only assume that 109 year old Edward is totally screwing with Bella's head. Why else would he date a vapid, boring, stupid girl like her?

My fiancee thinks it's driving the story--- you can build their mannerisms around what you need to happen in the story. "Characters are tools for your story; story is not a tool for your characters."

I picked mannerisms simply because it annoys the crap out of me when all the characters have the same dialect, habits, etc.

--
_{"Love is the total absence of fear. Love asks no questions. Its natural state is one of extension and expansion, not comparison and measurement."}
-Gerald G. Jampolsky