a) Visualization, and
b) What doesn't fit.
What do I mean? Well... here. Usually in this sort of exercise, one uses the names Bob and Alice, but I have a character named Alice, so we're going to call this nebulous new character Anna.
Let's get to know Anna.
We can start with something basic. What does she look like? Is she short? Tall? Average height? All of these things will come with various personality quirks. If she's short, she's unlikely to dig crowds, is likely to arrange her house in certain ways. If she's tall, she might be frustrated that it's hard to find pants in her size. Et cetera. How about her weight?
Is Anna conventionally attractive? Unconventionally attractive? Unattractive?
Is her vision impaired? If so, does she wear glasses or contacts? What does that choice say about her? If she wears glasses, is it a geek-chic thing? A lack of vanity? Laziness? (For me, it's mostly laziness, with a chunk of lack of vanity.)
How does she wear her hair? If it's short or she always pulls it back, she may be no-nonsense, not wanting it in her face. If it's aggressively short, she may have another reason; did something bad happen sometime that involved her hair? If she wears long bangs, does she do so to hide her eyes?
What is she wearing? Does it accentuate her figure, or hide it? Does it have any tells as to her likes and dislikes in music, TV, et cetera? Look at her shoes. Are they just something she pulled on, or is there thought there?
Look at her jewelry. Maybe she's wearing all silver, and mostly Celtic knotwork. This might also give you an idea of what sort of music she likes.
Now look at what doesn't fit.
Maybe all of her jewelry is silver, except one piece. Maybe a ring. Gold, set with a gemstone (which one? is this a birthstone, or a stone that might be important to someone else?), a little too big for her finger. Maybe when she thinks no one's watching, she plays with that ring, twisting it absently. Where did that come from?
And about that music. Let's take a look at Anna's iPod. Yep, there's the expected Celtic stuff (traditional? Celtic rock? Celtic punk?). But - what doesn't fit? Look for the song that doesn't belong there. Is it by Poison? The Glenn Miller Band? KMFDM? Focus on that song. Why is that there? What about that is so much a part of her?
Now let's put her somewhere. Say a coffeehouse. (What is she drinking? Mocha? Chai? Italian soda?) With a book. (What genre? What author? What does her choice say about her?) Suddenly, someone enters the coffeehouse, walks by her to place their order; this person catches her eye.
Well, first off - is this person male or female? If female, is Anna exclusively gay, or is she bi?
What does this person look like? What makes them Anna's type? Was it the hair? The clothes? Nice boots? Are they carrying a book? A laptop with interesting stickers?
They catch her eye. What does she do? Smile, wave? Look away quickly?
What would she do if they approach her? Or would she approach them?
All of these things will tell you about your character. Slo-mo this stuff - take a good look at who she's attracted to, because that'll tell you a lot more about her than just what she finds attractive. Take a good look at her iPod. Take a look at what she does when she's nervous - chewing on a pen, tugging her hair. Look at who she calls when she needs to talk. Is she close to her parents? Both? Just one? Neither? Why/why not? Siblings?
I could go on like this forever.
If you're planning on writing character-driven stuff, do this first. Learn them. Let them become people in your head. When you know their drink orders and whatnot... that's when they'll start telling you their stories, when the really organic dialogue will happen, when you can just sit back and channel this stuff.
Look for what doesn't fit. Because that's where the story is - the places where something is important enough to mar the smooth edges of their public face.