Characters in 3-D

In prepping to write this book, I read more books and articles on screenwriting, plotting, character arc and story craft than I can remember. As I try to put the advice of these pros into practice, I can only conclude one thing: I am on the learn-as-you-go plan. I learn from my mistakes and I make so many of them I must be learning a lot. Unfortunately, I haven't figured out how to take the good advice I am given and avoid the mistake in the first place. Ah, well.
I planned my protagonist, Sophie, as an obedient 14-year old German girl in 1938 and 39. Her character development included learning to step outside the rules of obedience in Nazi Germany and say 'No'. The farther I got into the story, the more I realized my main character needed more depth.
As a reader, characters that keep my interest are ones I can relate to, three-dimensional ones with positive and negative traits. Other than excessive obedience, Sophie didn't have any real character flaws, so I experimented with giving her one. I decided to give Sophie a secret, one that is threatening enough to keep her on her toes and keep the reader interested in how long the secret can be kept. When I did that, other characters sprang into action around her and reacted to her secret or their suspicions about it. The plot is growing itself around the secret and these 3-D characters like a vine around an arbor.
Yet another amazing series of discoveries on my journey.