on Jane Friedman site:

Humans are infinitely complex, and creating believable, three-dimensional characters in all their colors, textures, and nuances is some of the hardest work of writing. But authors don’t have to have godlike omniscience or an encyclopedic knowledge of psychology to develop fully fleshed, realistic characters. We all have an endlessly replenishing source of reference material: ourselves.

Learning to pay deliberate, meticulous attention to how you react to and handle your own challenges and triumphs, obstacles and demons can be rich turf for growing characters readers deeply relate to and conveying them vividly on the page. Start by noticing your physiological responses—your gut reactions—and follow them to the thoughts behind them—your inner life.