Building a character can be tricky business because, just like Real people, a character has to be multifaceted, they have to have a past (even though it may not show up in the book at all), and most of all they need to inspire emotion from your reader.
The best thing you can do is take some time to get to know each of your characters. Here’s a list of things I make note of for each character in an attempt to better understand them:
Who knows them
I have spreadsheet upon spreadsheet of character attributes for my different novels. Not only does it help you get to know the character, it also helps you to keep track of things so that your MC doesn’t end up with blue eyes on page 10 and brown eyes on page 219. And obviously I only do this for the main and secondary characters, though I do tend to go through and do a tidbit of stuff for those characters that aren’t in the books for long – in case they show up later ;)
How do you build a character?