Fictional characters should not be fictional. All humans share many characteristics; if your characters don't share these characteristics, how can your readers believe that they are human? A fictional character that survives five days without food must suffer horrible hunger-pangs, weakness, sweating, grumpiness; what would you experience if you endured what they did? If all humans would experience a specific reaction to one circumstance, then your characters must do the same ... or they are not realistic.

This requires two aspects: logistics and symptoms.

Logistics: Everyone has needs and expectations; for some people, these are the same. A wealthy person may have grown up having never gotten caked with mud; such a character could never casually walk through a city sewer system, no matter how badly the plot requires them to. A prude can't strip naked and seduce a prison guard without extreme emotional discomfort. This does not mean that your characters can't do these things; given a character who is a wealthy prude, I would do exactly these things: force my character to perform some act that repulses them. The inner struggle to do something that a character wouldn't normally do is what makes them real and sympathetic and admirable, but they can't do it easily; characters must suffer.

Symptoms: When a character's lover is taken from them, that character must experience jealousy. When a character tries and fails, then they must experience disappointment. When a character gets knocked out, then they must wake up with a splitting headache. When something happens to a character, they must endure what every other human would endure facing the same pain or humiliation.

Why must characters suffer? Because readers suffer. Readers suffer not being able to go out to dinner because they can't afford it. Readers suffer relationships that end. Readers suffer from losing jobs, and bosses that are having a bad day. All humans suffer, and dream about having the requirements to avoid repeating their sufferings. By reading about characters that suffer, and then overcome the causes of their suffering, it helps the reader empathize with the characters, transforming the characters into real people, and thus characters help readers deal with the sufferings in their own lives.

Characters must also be multi-faceted. I don't know anyone who is one-dimensional; every adult that I know has hundreds of likes and dislikes, quirks, styles, class (or lack thereof), favorite words, and a unique philosophy and outlook on life. Characters must be equally complex.

Almost every characteristic of my characters exist in someone that I know. Why? Because those characteristics are real. Real characteristics make realistic characters; I am always watching other people for interesting characteristics. Arguments are especially valuable; people mostly argue points of view that they are passionate about; by giving my characters those points of view, my characters become passionate. Passionate characters engender passion in readers.

But it must be written down! A character can love music, but if music never enters the story, then that characteristic only exists in the mind of the writer. As a writer, choose one of your main characters and write a list of facts about them that come from the story. Do the same for your best friend. Compare the lists: which is more complex? This will tell you if your character is more realistic than your best friend.

And, if your character wins the contest, then imagine how much fun you'll have telling your best friend how unrealistic they are.