Readers connect with interesting characters, not stereotypes. So what is the difference? Think of flat characters as first impressions that went no further. Interesting characters have desires, faults, and quirks. Like your friends they're not always easy to understand. They may contradict themselves and change their minds, but they remain who they are.

Writing means spending time getting to know your characters. It goes beyond character charts and outlines. No computer or pen is necessary. You need to listen for their voices, ask them questions, and have them play scenes out in your head. Oddly, characters can only tell you what they know. Once I had to ask a secondary character what secret was being kept from the main character. Another time a character had to undergo some therapy before I could understood why she did what she did.

And in answer to your unasked question--Yes, writers are little crazy.