Jill Bernard’s VAPAPO (Voice/ Attitude/ Posture/ Animal/ Prop/ Obsession) method is the most useful system for making characters I’ve ever come across.
She’s written a good introduction here.
I’ve created a little character toolkit called VAPAPO! It’s a mongrel of Marx Brothers, Commedia Del’Arte and Eric Morris. It’s: VOICE/ ATTITUDE/ POSTURE/ ANIMAL/ PROP/ OBSESSION! But let’s start at the beginning, just VAP — pick one at the start of the scene, a voice, attitude or posture. Then you hang onto it and voila! You have a solid character. What’s nice is if you lose it in the middle of the scene, you can just restore that voice, attitude or posture and you’re back on track. What’s also nice is that no one can ever steam-roll you. If I start a scene with a VAP choice and you say “Hi Mom,” you haven’t derailed anything for me, I can be an angry mom, or a car-fixing mom or a slow-talking mom, no problem, and so much more interesting than the normal bland mom choice. You’ll find also that when you pick one, you get the others for free.
Jill’s Small Cute Book of Improv can be ordered from here. Let me but assure you that it is as small and cute as it claims to be.