Characters are your story. They plan the subplots and drive the plot through their mistakes and their moments of brilliance. So how well do you know them?
You want your characters to be memorable. You want your readers to understand what makes them tick and maybe wish they were real enough to invite to dinner. So how do you take a figment of your imagination and flesh them out?
One of the best ways is to really delve into discovering who they are. You can do this by writing detailed character descriptions, writing a brief journal in their own words where they talk directly to you, and so on. JK Rowling has reams of pages detailing all of her characters, even the minor ones and some that didn’t even make it into her Harry Potter books. She knew who they were related to, what they liked to do in their free time and their flaws. When it came time to put them into a book, she knew enough about them to give you insight with a few select words. James Frey, author of several writing books, such as How to Write a Damn Good Thriller, is an advocate of the journaling option. He gives several examples and comments about how sometimes the characters surprise him by what comes through in these pages. By writing these pages in the voices of your characters, you can really get inside their heads, which will make it easier for you to put them on the page realistically in your story.
Yes, this is extra work, but it pays off in your story. No one wants to read about a cardboard character. Do this with your hero, your villain, and any other characters that are important to the story. You’ll be surprised how much easier it is to put them on the page and have them do what you want them to do in a way that is organic to them. You’ll understand intimately what motivates them and you’ll know their flaws and their good points. This will all come across in your story, making the characters come alive to your readers. They’ll fall in love with them, fear them, or feel the loathing or pity you want them to feel for each particular character. Try it. You may be surprised at how well it works.