1. Don’t let all your characters sound the same. If two characters are indistinguishable, they may as well be one character.
2. Don’t let one character bang the same note all the time. Characters that have the same range as a triangle in a brass band soon get tiresome, and worse, unbelievable. You aren’t betraying the character’s traits if they are angry yet occasionally sombre. Even wackos in the psych ward take a nap from time to time. Your characters need to act like people, not cartoons.
3. And please, don’t let smart people do dumb things to make a plot work. When you find yourself doing that, you need to step back and think about the plot problem so you can write around it convincingly.
A conclusion is where somebody got tired of thinking.