You can start by checking out my Meta Guide webpages, “100 Best Animatronic Head Videos” , “100 Best Robot Head Videos” , “100 Best Hanson Robotics Videos” , and “Talking Heads (Draft)” .
There are two aspects to this project, internal and external. In fact, I usually recommend people create first a virtual talking head, for instance in a virtual world, such as Second Life. Second Life is really a great testbed for protoyping robots. A virtual prototype basically covers all of your internal workings. Later, you can apply the virtual prototype to an external or robotic talking head. For this, you can go with a generic robotic head, and simply dress it up after the fact as the character of your choice.
So-called rear projection talking heads seem to be the current favorite; however, there are also “virtual manequins”  available, though perhaps something of a compromise. To do this well is actually quite a big and potentially costly project; however, I believe that you can achieve satisfaction with just the virtual prototype phase, and get a much clearer picture of what you want and how to get there.