If anyone is offended by the blog title, I apologise in advance. It's all my friend's fault, because she suggested the title when I said I was going to take the conversation about characterization I had with her and make it into a blog post. She cuts herself short, of course, but the title stuck.
First of all, I loathe a story which starts off - or in any way includes - a description dump. Please don't present your main character, or other characters, by using one or several paragraphs telling me how they look, what they've done in their life, what they will be doing, what their family is like etc etc. Want to tell me who your character is? Show it through action! Interaction is the best way to understand people - it's in the approach to others you reveal your own personality, prejudices, interests.
Remember that your characters are more than their looks. Looks are usually the first thing we judge in real life so it makes sense that it's the first impression we get from our characters. But as we travel down the path of the story together, looks aren't the only thing we notice. On a side note, our appearance is influenced by who we are. Confidence is attractive. A shy person might hunch together and look short. Now, let's stop thinking about looks - it makes me self conscious! So, who ARE your character? Which heartbreaks did she experience - and which ones did she cause? Or did his father berate him so much about sport he came to hate it? Is she allergic against her wolf-deamon? Your characters' history and backgrounds are directly linked to their behaviour today.
So what are the reasons why? Let's say your character hates celebrating her birthday. Why? Her father died on her 12th birthday and she doesn't want to be reminded of it. Or a less serious example. Your character hates Mr Right (well, Mr Right-to-be). Why? Because he has the same blue eyes as her first bf who dumped her after convincing her to have sex.
Note that you don't need to write everything into the story (although readers usually seem to enjoy these tidbits - I sure do!). But the fact that you know helps you realise how your character would act in other situations. Remember that your POV decides what you can relate about your characters. A limited 3rd person will not allow you to tell anything about the surrounding characters that your main character doesn't have a reason knowing.
Tying into that which isn't said - what a character doesn't want to tell about itself is usually the things that defines them the most. The secrets and hurtful experiences often affect our actions and reactions - the same is true for your characters.
Another thing that I often notice when people describe their characters are substated facts (eloquently named by me at this very moment). These are traits that I perceive automatically when another trait is mentioned (this is all due to prejudices, and can vary from person to person). For example, if you say "blonde" I imagine that person having blue eyes. If you say "fire fighter" I think "brave", "tells jokes" includes witty, fun, wanting to be the centre of attention (etc). So, focus on the things about your character that aren't already understood through other traits.
This ties in to the one idea that has helped me the most - imagine your character portrait being a caricature. When it comes to painting, an artist can immeditely relate a person's identity by focusing on a few distinct traits and then just be sketchy about the rest. Which parts of your character are the ones that separates him or her from everyone else?
Another mind-game is to remember that every effect has a cause (or several) and every cause has effects. For my part, I usually experience the effects first, and the realise the cause. That means that I see how my character reacts to something, and then I figure out why he or she reacted the way he/she did. Examples:
Cause -> effects (he almost drowned once -> he hates bathing/he has nightmares/he panics when he has hard to breathe)
effect -> causes (she's afraid to be alone -> she doesn't like silences/her brother scared her once when she was young/her own thoughts freak her out/her granny has been telling her too many sinister tales).
As this became such a long post, I'll save one important character developer for a later date - The Conflict. As always I appreciate your thoughts on the matter. Also feel free to give me ideas on what else you want me to write about.