My main characters tend to be flawed and powerful, yet powerless in their own way. I like to give my protagonists something to protect–a person, secret, etc.–and then put that something in the line of fire within the first two chapters.
For antagonists, I love to make them evil with a vein of good. A murderer who values his family and would never cheat on his wife. Things like that. I like that writing adage: The villain is the hero in her/their/his own story. Learning that made my stories so much better.