If you're a fiction writer, chances are you fall into one of two camps: those who find it easier to come up with characters than plots, and those who find it easier to come up with plots than characters.
I sit firmly in the first category. I love to create characters, but it's harder for me to come up with things for them to do that build a cohesive (and hopefully page-turning) story that someone might actually want to read. I tend to have a number of fascinating personalities who sit around drinking tea and exchanging wisecracks and verbal volleys. But--as has been pointed out to me by more than one editor--nothing actually happens. It's only with great effort that I get my characters up and moving and into the kinds of scrapes and jams that make for good fiction.
You might have the opposite problem. You might be a whiz with car chases, explosions, shocking surprises, spine-tingling suspense, and all manner of derring-do. But this exciting world might be populated by cardboard characters with no depth of personality, or any qualities to make the reader sympathize with them or be emotionally affected by the story.
If your fiction writing tends to lean too heavily one way or the other, I recommend the book Plot Versus Character by Jeff Gerke (Writer's Digest Books). Gerke writes as a plot-first novelist who developed a method for creating characters using, among other things, the Myers-Briggs Personality Type Index. As a character-first writer, I found tremendous help in how to develop characters in such a way that intriguing plot lines naturally follow. I no longer have to try so hard to dream up situations to put them in, or worry whether or not a particular action seems "out of character"--unless I want it to be.
If your writing is already well-balanced between an absorbing plotline and intriguing characters, then you probably don't need this book. But if you need help with one or the other, it provides a fresh approach that might just work for you.