Excellent characters are the staple of a good book. Without them the book is just words on a page. While I love the intricate worlds in a sci-fi/fantasy novel, it's the characters that keep me interested and that will make me buy more by that author. Dune is one of the most riveting books I've read but not just because Frank Herbert created an unforgettable world, because he created Paul Muad'dib. Consider your favorite book, what did you love the most about it? Chances are the answer will be one of the characters.
So how do we as writers create characters that will keep not only ourselves, but readers glued to the page and wanting more? One of the most populars authors of all time has the answer; Stephen King. His fans will tell you they love his books because his characters are so vivid and realistic. Readers need to be able to relate to characters, to sympathize with them, love them, or hate them, anything so long as they're passionate about it. King's characters are notoriously flawed and utterly human. That's what allows readers to relate so well to them because what human isn't flawed? Don't answer that. But you get the idea.
To breathe life into your characters you need to get to know them. If you have a plot first then it lays the ground work because you know somewhat how you want your characters to react to situations. Go a step further. All over the internet there are character development questions and exercises. Find one of these you like and take the time to fill it out. You'll be surprised what you learn about your character when you do this. It will give them depth and make them much more interesting to you and your readers.
Here are a few links to character development exercises:
100 questions about your character: http://www.gather.com/viewArticle.action?articleId=281474976908598
45 questions about your character: http://character-development.suite101.com/article.cfm/developing_memorable_characters