You love to create new characters and find that one of your favorite parts of writing is developing the individuals you have created. However, you know that you can’t just make a person out of thin air, as they will likely need to be connected to the world around them unless your story specifically calls for them not to be.
Rounding out your main characters means taking the time to give them a full personality, and a life that makes sense. It means making sure they are believable characters, and that the reader will be able to identify with them in some way. Your characters will need to seem realistic in the setting they are in and interact with the world around them in an appropriate way.
Characters that are fully developed will have an impact on the world around them, and the people and places they spend time around will impact their lives as well. They will enjoy the company of certain people, they will feel upset in certain places due to past events that have taken place there, and they may even show different sides of themselves in these different situations. Some settings and minor characters will be largely involved with the events your character takes part in or they may have a small, but important influence.
The goal of rounding out your main characters is to take the time to find a way to bring them into the story. Most of the time your characters will have lives, habits, obligations, and friends they have experiences with prior to your story even beginning, and it’s important that these are addressed in some way. That said, you want your references to be relevant to the story. Your own life is an accumulation of experiences, and your character’s will be as well, but that doesn’t mean that each detail impacts every decision he or she now makes.