Long after you've read a good novel, the individual scenes might fade in your memory but the characters will stay with you for much longer if the author did his or her job correctly. In fact, this is one of the necessary things that makes a good novel—characters that are immortal in the reader's memory. It is only through these immortal characters (think: Scarlett O'Hara, Don Corleone) that the novels, themselves, become immortal. They are the lifeblood of the work and without them, it would not have achieved nearly the same level of success as it did with them.
So how does a writer create characters that readers love? Well, it isn't easy. A character that remains immortal in the mind of the reader is likely a product of a lot of work on the author's end to make that character what he or she is. Just as with outlining the plot of the novel, outlining a character takes considerable time and thought. If you want the same for the characters of your novel—particularly the protagonist, try following these steps.
Give them idiosyncrasies
Let's face it—all people have idiosyncrasies. Sometimes it takes really getting to know an individual before we discover what those idiosyncrasies are, but then again, sometimes even the casual onlooker can see them. These can be anything from a nervous tic, to a strange gait, to eating habits that range from overly compulsive to simply bizarre. Having idiosyncrasies is what makes us human. It gives us uniqueness and distinguishes us from others. The exact same effect occurs when you give your characters idiosyncrasies.
One good way to do this is to take a few hours to observe others closely. It might be friends or co-workers or family members, but observe them for a while to see their idiosyncrasies. If you are observing someone you know well, then you probably already know many of these. However, close observation is always a good tool to discover more. Watch how they walk, how they move their hands when they speak, how they respond to noises or how they react when angry or startled. Make notes about these traits and use them (or versions of them) for your characters.
Give them hopes and fears
Giving your character hopes and fears is yet another facet to making them seem real. There are several ways to do this, but most of the time, authors use an interior monologue through which the character reveals them to us. This can be done through memories of childhood, reactions to events that take place in the story, or even through the character's own dialogue.
Giving your characters distinct hopes and fears has another benefit—it allows you to make the plot more intricate and deepen characterization. For example, imagine a scene in which a particular character faces something he has feared since childhood. You can use interior monologue combined with external dialogue to incorporate dramatic irony as a literary device: the character might be reacting one way outwardly while thinking something entirely different inwardly. This setup has multiple benefits, including revealing your character's fears—the fears that he or she keeps hidden from the rest of the world, even hidden from the other characters. This simple step gives your reader the feeling that they really know this character; they understand because they, too, have fears that they hide from others. And that's exactly what you want to achieve if you want the character to stand out in the reader's mind.
Make them larger than life
Your characters (at least your main ones) should be larger than life. They can be regular, average Joes but they need to do something that is extraordinary. When someone picks up a novel to read, they do it to escape—escape their everyday life and the mundane, escape a world that is ordinary. Very few authors have managed to write bestselling fiction with characters that fail to do anything extraordinary. It is this element of the extraordinary that makes the novel such a great escape to the reader.
Give them history
Our unique histories are a large part of what makes us who we are. In the same sense, giving your characters history will add depth, making the character seem more real in the process. This history should be first created when you are outlining your characters, but can be fine-tuned and reworked as you progress in your writing. Intertwining characters' histories is also a great way to add depth to your characters and plot, making it more complex and interesting.
Give them flaws
One of the most fascinating parts of a hero's story is revealing his tragic flaw(s). Through this, the character becomes more real, because humans are indeed flawed. Think of it like this: people who seem to be "perfect" are usually quite boring. It is the imperfection of others that attracts us to them, and their quirks make them more appealing. The same is true for your characters. Let them be messy or disorganized, let them have a temper, let them have insecurities, or let them be haunted by a mistake they made in their past that has had repercussions on their lives—even your protagonist. These traits will give them depth and make them more interesting in the process. Most especially, these traits will help readers identify with your characters, which is exactly what you want to happen if you intend to write bestselling fiction.