("no you didn't. Did it of my own volition")
This ties into a recurring unkillable theme in sf and indeed fantasy - the creation that goes out of control. Not only does it reflect in stories but it also comes full circle on authors and bites them in the behind-regions, because their characters become self-willed and recalcitrant (I have for sale a lovely new line in character whips, crops, martingales and, for the ones that really won't listen, the electric cattle prod. Forget the naughty step. It doesn't work on fictional characters, and besides, it's cruel and unnatural. Available today, if you call now, at our special promotional prices... but wait, there is more...).
Sadly, this is the natural way of things with well-developed characters. They become self-willed, get a+ for their Turing tests, and blunder through your carefully crafted plot like a drunken sow through a ladies tea-picnic, gobbling delicate crustless cucumber sandwiches by the trayful, stepping on china, ignoring screeches, and then decide that Great Aunt Agatha really is the sexiest boar they have ever met -- which just proves that rampaging characters too can be right.
You actually can't restrain them. While this may ruin my newest get-rich-quick-scheme (more inspired than galvanic buggy whips or the pornographic rabbit trap) nothing actually works. If you try, you'll ruin the book. You have to give them their head (and not necessarily on a platter) and vaguely try and herd them toward the less breakable parts of the plot furniture. And the worst of it is that not only do they take over your head, (which can be extremely dangerous. Do not, I repeat, NOT associate with me while I think I'm Benito Valdosta) you have READERS starting to mutter "I am the finest swordsman outside of France!" puzzling their nearest and dearest quite a lot.
These are the characters that you remember. Not always your favorites, but the ineradicable - sometimes not with best brain-bleach. They keep coming back on you. And, it appears readers come back to them. (It's difficult when the author gets demands for yet another book with Miles, or Ariel or Biggles in it. But not as difficult as not being paid.)
These characters are born out of primordial soup... well, the brains of writers, which is much the same thing. And the one thing that provides that final galvanic spark... isn't known. But modern science, with tireless research or at least an infinite numbers of monkeys have established that you need LOTS of bits. Now, we seem chock full of authors here. When you 'build' a character on your sla... uh in your mind - what do you put in?
Give! Maybe we can identify that crucial element.
Mine of course is attitude. And hair. I have never had a character come to life without knowing about the hair. Even those without it.
Well. I showed you mine. Show me yours?