Most Helpful Girl
Avoid too flat characters. Don't make a hero that is all good and never makes mistakes, make a hero that changes, that can grow through experience and from mistakes. Don't make a villain that's bad, make a villain that we can understand, that behaves some way because they think that's not wrong or thinks is the only way.
If it's not a serious mood novel but for comedy, then over use flat characters. The overexageration of their flatness can be hilarious if done well.