Here are more principles of theater that I think can be very well applied to creative writing. Objectives, conflicts and tactics make for exciting theater and characters, and it can do the same for the characters in your book.
Objectives: If your character doesn't want anything or isn't trying to get something they become flat and boring. Its not exciting to watch someone sitting there being content. All your characters need more general, overarching objectives that lead them through the whole book. But in every scene your characters need much more concrete, specific objectives: she wants her husband to admit he's having an affair--he wants to leave the house--she wants her sister to let her borrow the stunning green dress. The objectives are usually most successful when they go through another character. They need to be so concrete that there is one action that will signify when the objective has been obtained (leaving the house, admitting an affair, getting the dress). Then once you've got specific objectives, take the next step: raise the stakes. Make every objective life or death. She wants him to admit to an affair, because she's having an affair and needs to justify herself. He needs to leave the house because if he's late for work one more time he'll get fired. She wants her sisters dress because the boy she's liked for three years finally asked her to a dance and its the only dress she feels beautiful in.
Conflicts: These are obviously what is keeping your character from getting their objective. The most exciting conflicts are another characters opposing objectives. He won't admit to an affair because he wants to keep having it. He can't leave the house because his wife won't stop accusing him of having an affair, and leaving will just increase her suspicion. She can't have the dress because her sister was planning on wearing it that same night. Make sure the conflicts are legitimate obstacles. And don't be afraid to pile them on. Maybe while they're arguing about an affair the dinner starts burning. Maybe one of their mothers will be coming in five minutes. Maybe its July in Texas and the air conditioning is broken. Maybe mom already told her she's too young to wear a dress like that and if she hears them arguing about it then both of them will just get in trouble. Just keep it coming.
Tactics: For me tactics are where things can get really interesting. Tactics are the ways the character tries to get their objective. They need to be concrete, specific action verbs: seduce, anger, threaten, guiltify, excite, pacify, flatter, the list goes on and on and on. What makes things interesting is when you use tactics that are completely unexpected. What if she tries to seduce him in order to get him to admit an affair? You can play with these in so many ways, and tactics are what make your characters unique. Make the most of them.