Every book I write is sort of a self-inflicted class on how to write a book. Every time i have new insights as to what I'm doing wrong and how to improve it. Sometimes I am just banging my head against a wall, but then when I have a new insight, it's such a magical moment, I feel like I can almost see a message appear above my head: "You've gained a level! Subtlety +1! Character Development +2!"
So here's a few things I've been learning lately.
First, here is a tip from my Twitter, a couple of people seemed to like it, so I'll mention it here too. For giving quick life to minor characters (you know, like "waiter" or "girl on street") keep some books of portrait photography in your library. While writing my Weimar Berlin-ish world I've been obsessed with the brilliant works of German photographer August Sander. He photographed people of all classes and professions. I couldn't afford the complete seven-volume set of his work but I bought a pretty decent collection and it's so great to just pull it off the shelf and immediately flip to bohemians or old farmers or whatever.
Sometimes it isn't that you don't know the character well enough, it's that you've given them the wrong thing to do. I've been realizing with the Dark Metropolis sequel just how important it is to give characters scenarios in which their most interesting self can shine. In draft #1 I had theoretically interesting scenarios, but they weren't the right ones for some of the characters. My protagonist Thea is a slightly religious girl and she wants to turn to her religion for comfort but she is also questioning her faith. In the first draft, she met a character of a different religion and reacted judgmentally to him. This was true to her character, because right now she's freaking out as she deals with a lot of big thoughts. But it also made her seem narrow-minded and unappealing. I switched her circumstances around to where she was in a position of strife, trying to turn to religion for comfort and butting up against her new, shaken view of the world. Same character, same struggles, but this was a more likable, relatable position for her.
If your cast has gotten too big all at once, find a way to break them up. At the end of Dark Metropolis I have a lot of people together at once. So as book 2 begins, they're still together, and...they were getting lost. It was like trying to meet characters at a really stressful cocktail party. I found a reason to break them all up again and the story got much better.
Torture your characters in the book, to the point where even you might be a little uncomfortable. I have often heard advice to not be too nice to your characters, and don't be afraid to do bad things to them. I have always ignored it because I LOVE torturing my characters! Well...or so I thought. I realized I mostly torture them before the book even begins. They all have tragic pasts, and they are still dealing with the ramifications of their pasts, but I realize I rarely do anything shocking to them in the middles of books. Erris = already an automaton. Nimira = already in a low position. Alan = has a secret in his past that has been hindering his happiness. I have never made a huge game-changing tragic thing happen to a character in the middle of a book, although Magic Under Stone was a tad better. So. Gloves are off now, characters. Be warned.
Characters that have been unloved are a classic of literature. I certainly wouldn't tell you to put away all your downtrodden orphans and whatnot. But I've found that unloved characters often give me a lot of trouble. I had this situation in Magic Under Stone when I wrote Ifra as having a loveless childhood, raised only to serve others as a jinn and accept his fate, but he actually showed up in my dreams and told me, noooo, he had actually been able to spend part of every year with a farm couple who treated him like a son. He drew strength from this throughout the book and it made him a lot less emo and angry and more strong and sympathetic. I realized I'm doing it again with Freddy in this book, and I want to go back and make his relationship with the man who raised him more nuanced. Good thing I still have a revision pass to go on book 1!