Change vs. Growth: Apply this to your characters and/or your life as you see fit. Don’t let yourself (or your characters) get caught up in “I am who I am and I’ll never change for anyone” that you (or your characters) stop growing. Don’t compromise your values, but be open to new experiences. While certain aspects of you will remain constant throughout your life, who you are and who you’re becoming are usually in flux, especially for younger people. Consider your character’s values before they make decisions, and let those experiences be real. Make sure it’s obvious when they do something because they want to vs when giving in to outside pressures.
Conflict advancing the story vs. people who avoid conflict: Bridge this gap between reality (people usually avoiding confrontation) and fiction (confrontation and conflict are necessary to advance the plot) with a few techniques: let unsaid things be conflicts, let things come out in smaller, subtler ways, and bringing out the big conflict at just the right moment.
Should you base a character off yourself? You might know your self-based character very well, if you’re a self-aware person. But it can be easy to feature our best sides, creating a Mary Sue type. Write in some distinct differences to avoid this pitfall. Let your character grow in her own way. If she starts acting differently, try letting her, and see where she goes. Also, try to play up her flaws so she’s well-balanced. We all have things we don’t like about ourselves, and that’s what makes us real and interesting.
Simplify Character Building: The process of character building can be a lot easier by returning every now and then to one simple question. Ask your characters, “What do you want?”