Now that you've created the skeleton for your creature - you've developed an outline using the 3-Act structure - it's time to move on to the internal organs. At the heart of your story is the protagonist - your main character. Your protagonist is the center around which all the activity in the story revolves. In "Gone with the Wind," the protagonist is Scarlett, and even though I wouldn't really categorize Rhett as a secondary character, given his importance, he still takes a back seat to our feisty southern belle.
As you develop the characters for your book, consider keeping a "character worksheet" in which to list such details as eye and hair color, age, family dynamics, etc. Keep this worksheet handy so you can refer back to it as needed. Start an Information File for your book. The Information File is different from your Ideas File in that it pertains to a given project. It's where you put your character worksheet, all the research you've done for your story and other pertinent details. If you keep all of this information in one place - a 3-ring binder works well - you can access it quickly when you need to look up something. You can access the character worksheet that I use, and feel free to make as many copies as you wish.
How do you create compelling, real-life characters that jump off the page? You give them a personality. Your characters have flaws - even your hero/heroine - just like real people. No one is perfect, and, frankly, perfection is boring. In addition, your reader must be able to identify with the protagonist. The reader steps into the story, feels like he's become a part of it, and if he can't feel some sympathy for the main characters, he won't care what happens to them.
So, even though your hero may be deeply flawed, he must also be seen as sympathetic. How do you do that? By using an old trick called "Save the Cat." In recent years a book with that title has emerged, and though the book is about screenwriting, it applies just as well to fiction.
I'll give you an example: Your main character wakes up, hung over, in a strange apartment. He doesn't know where he is, but this isn't a new occurrence for him and the two-day stubble on his face tells him he's been out for some time. He gathers his clothes, sneaks out of the apartment and heads out in search of his car. Not a terrific beginning. Your reader wonders, who is this guy? But then, as your character is leaving the apartment building, he stops and takes the time to help a meowing cat out of a tree (thus the expression). Or he helps an old woman carry her heavy packages up the steps of the building, or pats the head of a small child... you get the idea. At that point your reader realizes that, though he's dirty and hung over, this is a good guy! So it's easy for your reader, from that point on, to care about what happens to him.
Of course your characters' flaws can be more subtle than a tendency for alcoholic blackouts. Scarlett's main flaw (she actually had many) was her selfishness, which, by the end of the book, she was taking steps to overcome. Your characters can be selfish or self-centered, short-tempered, even prone to shoplifting - just make sure the negative qualities don't outweigh the good. And, if possible, have him at least try to become a better person by the end of the book.
"If you will practice being fictional for a while, you will understand that fictional characters are sometimes more real than people with bodies and heartbeats"