Hey folks, here's another of those lovely "How to write" things that all writers love to write.

So that they can feel righteous.

Ducks rotten vegetables

In all seriousness though, the best way to make sure that your character won't come out 2Dimensional is to plan. This might sound obvious, but when it comes down to it, planning can not only save you a stackload of writers' block, it can also help you develop your storyline, as you stop to think, "Hey, so how WOULD (to take a cheap example) a poor woodcutter really react if he got told that he had three wishes?"

Well, it depends on the woodcutter.

Is he smart? Is he stupid? Is he cynical or naïve? Does he have an overwhelming desire for something/an addiction? Does he have kids? Has he ever met someone who had had an experience with wishes, was it good, bad or neutral, and how did he react to that?

All of this illustrates the seemingly blindingly obvious statement that must be considered in regards to any writing that contains a sentient object.

There are many different elements to any character, original or otherwise.

Here is my shortlist of so-called "rules" (note the quotation marks, I will get back to this) that I use whenever I start to brainstorm a figment.

Motivation: All characters have an agenda. It might be love. It might be a desire for world domination. It might be hunger, hell, it might be a strong dislike of beige carpets or a hankering for fudge-ripple icecream. Whoever, or what ever your character is, there is a reason for their actions. Whether or not you reveal this to the readers is up to you as a writer (enigmatic characters done properly can be a lot of fun,) but YOU THE WRITER need to have it clear in your head why the hell Ms. Blahdeblah just made off with Mr. Whoeveryouwant's pet llama.

This is an area where backstory can be fun, but for the love of god, don't infodump. A flashback that actually adds something to the story can be good, but if it's not necessary, then scrap it.

(Basic) Description: This might sound obvious, but you gotta know what your character looks like to write about them. If you can't get a clear picture in your head, then describe a real person you saw on the bus that morning, or a celebrity or something. Think about the first things you actually notice- the classic mistake any amateur writer might make here is to assume that hair and eye colour are the first things you spot.
Are they really?
Does your character have an idiosyncratic look about them? Their smile? One eyebrow might be thicker than the other, or they might have a large mole on the back of their hand. The sky is the limit here, but don't go overboard.
This might be a shocking revelation, but you can often get by with only the barest of descriptions. People will always make up ther own mental pictures of what your characters look like anyway, so unless the appearance of the character is somehow important to the plot, it might even be worth not going beyond some vague reference to age and general demeanor.
So in short, with physical descriptions, often less is more.
This can also have the added bonus of making your story colourblind. Never underestimate the value of this to your readers.

Background: Okay, I know I brushed over this before under Motivation, but there are other aspects to background that I didn't cover.
The first of these is family/friends.
Ever wonder why so many orphans, single children, single parents, etc. exist in stories? If you've ever written much of anything, then you probably know where I'm coming from when I say, "because writers can't be arsed".

That's right, people. It's because writers are slack.

Everytime you introduce a new character, the number of relationships increase. (I know you're all saying duh at this point, just let me finish). Say you have three people, for simplicity's sake, let's call them A, B and C.

So A has a relationship with B, and a relationship with C, and a relationship with both of them.

WHAT?!

Think about it. When you hang with a person, you might act a certain way with them. You share a certain history, a certain comfort zone of reaction, blah blah blah. When you hang with another person, same situation, only, the way you relate to them is slightly different, so the comfort zone of reaction (for lack of a better phrase) will likewise be slightly different.
But wait! These two people hanging together without you (yes, it is possible, for all you narcissists out there) might have an entirely separate dynamic going, an entirely separate comfort zone of reaction. This exists even if the two have never ever met, as they are reacting to each other as they would react to a stranger, albeit one they have assumedly heard of through You.
So if three people are hanging together, then there should be a compromise between the three separate relationships.

Add more people to the mix, and this increases exponentially.

So before you decide to add "Token Cute Little Sister" or "Token Ex" stop and think. Do you want this character to be a cardboard cut-out for your precious OC to relate to, or is there going to be some actual interaction that might be interesting to read going down?

Because if you actually want your OC to be worth reading, you need to consider these things, and consider them well.

This isn't to say that there isn't a place for card-board cutouts. They can be fun to play with in satire, and they can act as catalysts for more interesting developments in the storyline.

BUT FOR THE LOVE OF GOD MAKE SURE YOUR OC IS NOT ONE.

Bottomline: I don't give a shit about how "pretty" "witty" or insert cliched OC trait here your character is, if they don't actually interact with other characters in a natural way, then they are automatically a Sue.

And when I say "natural way" I don't just mean something like male characters having instant Neanderthalism imposed by a "hot" female (or male) of the species.

Your character is like an actor. They have to convince me that they're for real. You have to ask yourself, is your character that kid who auditioned for the school play by reading lines in a monotone, or is that character a serious method actor who has done their homework by LIVING the character?

You don't have to overprepare (as this can be stifling, as somone mentioned earlier) but you have to be convincing.

Otherwise, just like any actor, there is no way in hell your character is going to get an honourable mention in anything but the tabloids.