Well, as you're one whole point of charisma away from being in a coma, I suggest not being able to talk in anything other than a mumble (half of the time not actually using words, just things that sort of -sound- like words), staring off into space blankly, repeating yourself alot (again, don't forget to mumble only), and rocking back and forth.
Edit: Oh, and never look directly at anyone. Always look off to the side, down at the floor, or off into the sky. And again, with the thousand-yard stare.
A cross between Senor Cardgage and an autistic, basically.