The answer is, whichever works for you. I've done both. Let's say you choose to begin with characters. Where do you get your ideas? Sometimes I'll base a character off of a movie actor or actress, or sometimes they'll come from a picture in a magazine. I've even been known to compile characteristics from different people I've met and make them into a fictional person. Each of these suggestions gives you a visual to work with, which for me, is essential to keeping true to the character.
A wonderful tool to access your character's motivations is the book Getting Into Character, by Brandilyn Collins. This book suggests you interview your characters until you know what drives them -- inside and out. By doing this, you'll then be able to develop various plots and subplots for your story.
Now, say you begin with a plot. Where can you find plot ideas? Many times, I'll form a plotline with a story I've read in a newspaper or magazine, and the idea branches out from there. There's an old adage, write what you know, and that's good advice to keep with you. Think back to an occurance that changed or marked your life somehow and see if a plot develops. You'd be surprised how many stories begin that way.
Now, say you begin with a plot. Where can you find plot ideas? Many times, I'll form a plotline with a story I've read in a newspaper or magazine, and the idea branches out from there. There's an old adage, write what you know, and that's good advice to keep with you. Think back to an occurance that changed or marked your life somehow and see if a plot develops. You'd be surprised how many stories begin that way.
Another option is to hold a brainstorming session with other writers or friends. I participated in one of these last month, and it turned out to be very effective. You can do this with as little as a vague plot idea or a glimpse of your characters. Share your mini synopsis in a round-table group discussion and see what ideas pop up. It's fun to see how one person's thought feeds off another, and it really gets the creative juices cooking.