It is portrayal of extreme goodness with extreme evil that makes the story believable and causes us to lose ourselves in the process. It was no wonder that the Russian authors such as Dostoyevsky, Tolstoy, and Chekhov are renowned for their craft. Each of them were supreme psychologists with the portrayal of the human spirit brilliant in its comprehension and complexity. Take Dostoyevsky's 'Crime and Punishment' for instance. Here, evil slurred side-by-side with goodness. The hero murdered. Yet insertion of dichotomy into the narrative reveals his honesty, inclination towards religion, and desire to be good.

Pierre Bezukhov, a bumbling figure, was no different in Tolstoy's War and Peace. A drunkard and a fool, he was likeable, loyal, and ultimate survivor and hero of the tale.

In no other story could the benefit of dichotomy be more clearly evidenced than in Anne Karenina. The narrative is rife with contrast. The heroine herself was an adulteress who was dedicated to her kids and reluctant to leave her home. Her...

Her husband, too, is a dichotomous personality: simultaneously devoted to his wife whilst sterile and aloof. The only one who seems a cardboard figure is Count Vronsky, and indeed he fails to hold our interest.
The ongoing and never-ending absorption by the novel indicates the need for writers to construct real-to-life figures. In real life, humans are a mix of emotions and personification of contradictory habits and passions. Humans, themselves, are a dichotomy, often incomprehensible and fluctuating in their contrast. The writer, the fore, who wants to succeed and haul readers into his narrative -- to make them laugh, cry, cheer along with them, and hope that the book will never end (and remember it as well as an Anne Karenina has been remembered) had better make his characters strut to life. And the way that he can do so is to inject dichotomy into his writings.

References

Dickens, C. Tale of Two Cities, Signet Classica, NY, 2003