One of the easiest mistakes a writer can make is not knowing who their characters are and why their story is important. Whether you are a plotter or a person who writes by the seat of your pants, as the author you must know your characters deeply. I've spoken about goal, motivation and conflict. I've spoken about starting the story in an interesting place. Neither of these things can be done if you don't know who your characters are. Very simply put, a story is about one thing, character growth or what is called a character arc. If the action does not change the characters, then there is no story--only a sequence of events.
So, before you begin, or even while in deep revisions, ask yourself. How does my character change? Who they were at the beginning of the story and who they are at the end must be different. Some formerly held principle in your character must change.
Many beginning writers will make these changes extreme. They will take a murderer or a prostitute and change them into a suburban mom who goes to church each Sunday. Then they wonder why the editor or agent didn't like their heroine. They say, "but I redeem her in the end." These kinds of extremes--while showing that the author is aware of character arc--are too blatant and leave the reader cold at the opening. Why? Readers like to be the point of view character. If that character is too cold, evil or calculating then a reader won't stay with that character to watch them grow. As an author you need to think of character arc in more realistic terms.
A simple tool to use is the W. Think about who your character is in the beginning and why the reader cares about them. Ex: Harry Potter-an orphan living with uncaring family. Now think of four turning points in their development. Keep them subtle. Ex: things go downhill for Harry with his family until the first turning point for his character-Hagrid from Hogwarts arrives. From there things get better. Harry learns he is more than a mere orphan. His adventure into a new life begins. But things can't stay wonderful or the story would be over. So then next turning point arrives. (Think W --down, then up, now down.) Harry realizes that he must believe in himself even when others don't if he is to help his new friends. The final turning point things turn up-but subtly. In the end Harry returns to being the boy in the closet, but he now knows he is more than that. And that carries him into the next phase of his life.
Each character in your story should have their own subtle W. Whether you plot or write by the seat of your pants, having character W's will help you build a better story.