In response to Christa's comment question, I imagine the character as a real person, and create in my mind their life story so to speak. I do character charts that are so detailed, I know what my heroine carries around inside her purse. Not all of what I know about my characters will make it into the book, but it helps me write their story.
Some authors do personify themselves into the character, so to speak. But I don't do that, it just doesn't work for me. I imagine them a seperate entity from myself. I do, however, try to get inside their heads and hearts and thoughts and motives and hopes and dreams.
So to answer your questions, I, being the compulsive organizer that I am, form lists.
There are a gazillion blanks I fill out. Some people can't create characters this way...by using character charts I mean.
I try to put them in scenarios and figure out how they'd respond, react, etc.
I often fashion them after actors or actresses so I can picture them, and better help my readers picture them.
The only way I know how to describe it, is I see them in color, in moving picture in my mind. I know their thoughts, the intent of their hearts, etc, etc.
If anyone wants examples of great character charts, there are TONS on the web. One I use very often is one Margaret Daley has on her website or blog. www.Margaretdaley.com
Navigate around there until you find her character worksheet.
Hope this helps,