The human voice seems to have two parts — high and low. You have resonators: in the head, and in the chest. Your sound changes depending on how much resonance is focused on which part of your body. You’ve tried sirens up and down, yes? And there’s the clunk when you move between the two. Well, here’s some quick exercises to make each stronger, and merge the two.
When doing sirens, make sure your breath continues strong to support your sound. While sirening, use your hands to pantomime pulling the sound out of your mouth. This increases the energy you’re investing in the siren, and enables your voice to make the bridge between registers more fluidly. You can also make circling motions, as though your breath were a mill wheel.
Use different registers as you speak, just every day. This might perhaps be by yourself to start with (we all talk to ourselves, yes we do). Use the register you are the least comfortable with! For me, I use my lower register much more. So my challenge to myself is to focus UP.
The exercises will have two effects: 1) your everyday speaking voice will be fuller, and 2) your search for character voices will be easier and you’ll find more possibilities. Enjoy the process, and practice often!
Here’s a few interesting websites:
http://www.thenakedvocalist.com/the-mixing-myth/ Long but interesting conversation.
http://www.voicecouncil.com/increase-vocal-power-with-mixed-voice/ This has videos, is more for singers but is a detailed explanation.