Arguably one of the most important elements of any work of fiction are the characters. No matter how interesting a plot may be, or how beautiful or humorous the writing style, if the characters are flat or otherwise uninteresting, your story will be a flop. Readers need characters they can relate to and understand; who seem as lifelike and realistic as possible. In other words, if the characters are similar to real human beings, a work of fiction will be far more successful.
There are many ways of accomplishing this, and one of these is through the natural growth of the characters. Human beings are constantly changing as they age, experience new things, and go through various stages in their lives. It would be highly unrealistic for a character to stay stagnant throughout an entire work of fiction. Whether they move forward or backwards in their development, a reader should be able to clearly identify how a character was at the beginning of the book, and how they have been altered by the end of the story.
When I began taking creative writing classes, one of the first things I learned was how to make characters either go forward or backwards. One of the things that readers find most interesting is a fundamental change that takes place in a character that will alter their perception of who they are. For example, a woman might think "I am a mild, shy and quiet woman, who works at the office and enjoys a simple life." All of a sudden, her husband is murdered, and the police are doing nothing to solve the crime, though she's positive she knows who the murderer is. This woman might now find herself taking control of a chaotic, wild event, and she will argue viciously with people in great positions of authority. By the end of the novel, this character will no longer have the same opinion of herself that she did before. Readers will simply love this.
Contrarily, a character might not grow at all. An old man might say to himself "I'm confident that I've lived a good life, and I'm ready to die in peace and be reunited with God." However, when he is diagnosed with cancer, he suddenly decides that he's going to fight to live. He begins to think of all the things he's never done in his life that he wants to accomplish, all the people he doesn't want to leave behind, and he understands that he isn't ready yet. In a sense, he might fear death, and it's only through being diagnosed with cancer that he realizes it. Ultimately, this fear turns him into an irritable, fearful man, and he dies with little dignity or happiness. He is not the man he once thought he was; in his eyes, he has become worst.
Try to examine your story's main protagonist. Does he or she clearly evolve? This might be shown through changes of opinions they once held at the beginning of the story, different or enhanced qualities they either desired or rejected early on, or a certain maturation that might take place in younger characters. This will be portrayed through their inner thoughts and dialogue, as well as their actions.
If we return to the quiet, shy woman from before, we can imagine how we might evolve her character. At first she is very calm and quiet, avoiding most social events and preferring to go home and spend time with her equally soft-spoken husband. However, when he is murdered and she desperately wants to put his killer behind bars, she might become far more outspoken. She is no longer afraid of social situations, daring to go up to a police officer and demand that he listen to her. Her inner thoughts might also be more bold or assertive. These changes in behaviour and thoughts will quickly be noticed by the reader.
Ultimately, a character that develops and changes in a piece of fiction is a character that readers can appreciate. Whether for better or for worst, no human being ever stays stagnant, and we're all constantly morphing and changing. By allowing our characters to do the same, we are connecting with our readers and providing them with lifelike, realistic protagonists and antagonists with whom they can relate, which will always make for a more interesting story.