When I first began writing for a living almost fifteen years ago, I believed that every story I told had to start with the plot first and then the characters. I felt that I had to know the story I was going to write before I wrote it. Now, things are completely different for me. I write in a very different manner.
I usually start out with thinking, "What topic do I want to explore?"
The way I came up with the idea for my book was that I sat around thinking, "Wouldn't it be cool if there was some way for me to get two pantheons to duke it out? I'd love to see the Norse gods fighting the Greek gods or the Egyptian gods fighting archangels."
That's how the story of Age of Immortality came to be. That's all I started with. I didn't know a single character. I had no plot. I just had an interest in a situation, and my next step was to fill in the details.
I decided which pantheon I wanted to start with. Then I figured out how my character actually met those gods, and then I began figuring out the details of my actual characters. I wrote about 8 chapters before I decided I needed to be more organized with my characters, so then I sat down with some note cards and wrote out all their major quirks and personality traits. Then I went back to writing.
I have used this same tactic in game design. While I was at GDC Online, I was playing with my husband's Android. He had some game on there with a bouncy cow that was trying to get up to an alien ship. The game play was very much unappealing to me, but the cow... boy, I really loved that cute little cow. So, I told my husband that I was determined to make a game about sheep. That's all I started with. By the end of GDC, I had most of the game design banged out. I did a lot of it during Raph Koster's talk. (He was great!) I was told afterwards that the girl sitting behind me and to my right was a writer from MMORPG.com, and she had spent half the talk trying to read my notes. I can't talk too much about the game yet, but the point is that I started with "I want to make a game about sheep."
P.S. The sheep pictured above is a screenshot from ATITD, not from my sheep game. My sheep are much cuter!