Your Characters - How do you Build them & Flesh them out?
Character development has be organic. A lot of the time, I will have an entire background of a character fleshed out before I begin writing him or her. These little details, although they may be unimportant to the overall story, does help bring definition to your character.
The best way to write a story is to establish motivation. If a hero/heroine is angry, then why? If they're seeking revenge...why? You can apply this question to every emotion and facet of your character and this gives depth to any situation. It's what readers want and what they expect. Characters should have flaws, whether or not they are physical imperfections or emotional, because scars give context. No one is cookie-cutter perfect.
So here's where I plug one of my books by giving an example. In my latest story Come With Me, the heroine Lainey has survived for a year in the apocalypse because she'd been alone her whole life. She grew up in the foster care system, never had anyone close, so when a virus comes along to wipe out humanity, she has the tools in place to survive by getting away from everything and every body. She doesn't need human interaction because of the tools she learned growing up.
When you write a story like that, you have to ask yourself lots of questions. Why was she in the foster care system? What career could she take on that reflects her isolation? Is she capable of taking a life? Does her background make her stronger in this area but weaker in others?
Another way to flesh out your characters is to place yourself in their shoes. If I were Lainey, what would I do?
The key to character development is really thinking about their motives. You can NEVER overthink. If your characters are real to you then they'll be real for your readers.