By reading Plot Versus Character: a Balanced Approach to Writing Great Fiction.
If you dream up intricate plot points, with high drama, and perhaps a twist at the end, you'll likely be less adept at creating believable characters. If you invent characters so real you expect to meet them on the street, you might find it hard to give them enough to do.
This newest Writer's Digest book is full of all the information both plot-first and character-first novelists need in order to take advantage of their strengths and sharpen their areas of weakness. Jeff Gerke, himself a novelist, editor, and publisher demonstrates each point with excellent examples.
If you are a plot-first writer, you'll discover that deepening your characters will increase the stakes, and may provide ideas to make your plot elements even more compelling. For the character-first writer, will use the dimensions of their characters to create far more riveting plots.
Part one of the book will educate you on creating characters that are memorable and three-dimensional. Gerke's Character Layers Chart is an amazing addition to a writer's arsenal.
Part two helps writers who struggle with ideas for interesting plots use Gerke's tools for building them. And part three helps to integrate both plot and characters to make a novel that is difficult to put down. You'll probably find Gerke's Plot and Character hard to put down.
For more info on Jeff Gerke:
His personal website, Jefferson Scott.
Gerke's speculative fiction imprint (with fantastic books): Marcher Lord Press.