Let's get right to the point. One of the greatest problems people face with the Inheritance series is the fact that the characters are lackluster and unnappealing. This has less to do with their personalities and more to do with the fact that they just aren't well-rounded enough.
I've seen many articles like this online, most of which suggest that in order to have a believable character, you should make a character outline. This is generally a listing of the character's qualities--their appearance, their quirks, their likes and dislikes, the way you would probably answer one of those "tell me about yourself" surveys that Diamondo likes so much.
Up to you if you want to spend that much time on every character you make...but personally, I think this is more entertaining than helpful, and can even be dangerous. It certainly does not guarantee that your characters will be believable or rounded.
Why is this?
Tell me, can words properly convey to me what you are as a person? Can you say "I'm witty, cheerful, get depressed easily," etcetera? It might give me an idea, but it's not enough. I would need to actually get down and talk to you, and even though in the end of our conversation I may not get the impression that "She's witty", I would probably be able to get an overall shape of how you are.
The same can be said for book characters. (This article ties in closely with the Show, Don't Tell! article available here). The thing is, when you start 'listing' traits, you are therefore feeling obliged to 'tell' the reader about the character, rather than 'showing' them who they really are. And oftentimes, these traits contrast greatly without rhyme or reason. God forbid you actually divide a character into his "qualities" and "flaws"...you will end up with a schizophrenic emo who goes in and out of things because of your whims.
So well...what do you do then? How do you create believable characters?
It's very simple, really.
Make them real.
Think of them as real people. Have an idea of what they are, what their personalities are like (WITHOUT listing it!!!)...their pasts, their present. And then, let God do what he will. Let them surprise you. Let THEM make their own decisions. I've often found myself sitting in the beginning of a scene asking a character, "So, what would you be doing at this point?" and the character replying back, "Well, if I was Sume, I'd help him out, but I'm not, so..."
Think of them as your friends, as other people, not as your creations. The worst mistake you can make is thinking that because you're the writer, you have the almighty power to do with them what you will. Well, maybe so...but you're not going to be writing anything worth noticing for a long while yet.
Do not make 'action reaction' things. Have a certain idea on why characters do what they do, but make it vague and complex enough, as it always is in real life. Most things you do, after all, are done because of the mixture of a number of things. Oh, sure, there's some traumatizing exceptions, like you being scared of cars because you nearly got hit by one once, but when it comes down to it, you can't say that you talk fast because your sister raised her eyebrow at you once when you didn't.
The same can be said for character development. Be aware that 'turning points' wherein a character completely changes is clichéd. Most people change gradually, over time, and they tend to have momentarily lapses of their old behaviour anyway. Most people also don't make a long speech and then change after that. Some MAY, but they don't make for believable characters (and you as a writer shouldn't be making excuses anyway).
Show characters what they are, and don't make the mistake of mentioning the dreaded personality traits I said earlier, even offhandedly. Don't say, "My God, Yallie is so witty!" Rather, show it, through Yallie's actions and dialogue. Don't say, "You're so hotheaded," SHOW the character being hotheaded and DON'T refer to it (i.e. if you point something out so very often the reader is going to say, "GEE, I get it already," and is going to wonder why you thought it was necessary to do such a thing).
Also remember that characters do not necessarily have to be drastically "different" from each other. Don't fall into the trap of making the aloof character, the quirky character, the good-hearted thief, etc. As with stories, people are "original" because of their own personal experiences...and so two aloof characters will be different and should give a different feel because they've had different backgrounds, different religions, or at the very least, well...different lives.
Let the characters live off the page. This is another important one. Do not make them move for the sake of the plot. Think of their personal motivations, why they're doing what they're doing, what they want to accomplish out of it. Think of them taking great efforts to "live"--a character who doesn't feel the pang of hunger when in a dungeon and would rather spend time thinking about how the Empire is so cruel and the slaves should be set free is not very believable. Think of them having desires, often selfish; of wanting to have a good place to rest rather than saving a kidnapped princess they've never known. Tempt them, distract them, make them as you and I are.
