Using "off the shelf" parts to make a life-sized human head that moves in a life-like manner and can be made to move its mouth in sync with sound, or even manually using a joystick. Total hardware cost target: ~$200
You can also use what you learn here to apply to ANY other type of bot that uses similar dc motors to control movements (WowWee "Chimp Alive" for instance).
Potential uses: Uncanny Valley Chatbot, Halloween/party prop, Talking alarm clock, or Robotic Youtuber!
Tools: screwdriver, wire clipper/stripper, soldering iron/solder, hot glue gun.
Hardware: Arduino pro mini ($5), motor driver ($10), Sparkfun Spectrum Shield ($30), audio board ($10, optional), audio cables ($10, for audio in and out), misc solid and braided wire. If you are not comfortable with the small form factor of the Pro mini, you can always use a costlier, relatively large and easy to wire up Uno (the Sparkfun spectrum shield is made to fit on an Uno). Also, using a board with a USB connector makes programming much more convenient (no FTDI adapter needed).
Software: you will need to know how to program an Arduino at a very basic level.
A WowWee "Elvis Alive" was used as a human head. The Elvis is a very complex robotic head that has DC motors independently controlling head "roll", back and forth movements, eyes, upper lip, and the jaw. While an "Elvis Alive" bot can almost always be had on ebay for $250-400 "new", they do occasionally go for ~$100 for used ones. The reason I like the Elvis Alive is that it has an interesting "Alive" mode where it makes random movements like a strung out pop star, and says canned audio clips of Elvis himself.
As mentioned, the Elvis has an "Alive" mode (hence its name "Elvis Alive"!) which does basic random head and eye movement. The head will also track left and right using the IR detectors on the left and right front pockets on the bust.
They key for this Instructable is hijacking the jaw servo only, so we can make the bot talk in response to audio input of our own choosing instead of the factory canned audio clips of Elvis. Some things that could be used for audio input include, of course, TTS from your computer (email, weather, rss feed news, Twitter, Facebook) or simply voice clips of who/what ever you please. You could even make it speak in response to input from a microphone.
If you are lucky you can get one that is still in the original box and has the shipping strut still attached (plastic post that hold base and head rigid during transport). But no matter what you get, this IS a hack, so you can make do with whatever you end up and who knows, it may lead you down an entirely unexpected path. Anyhow- once you have a bot, test it to see how well it still works. These are old bots (2007-ish), so even "new" ones may not be 100% functional. You will need to have the 9V AC/DC 2.7A adaptor "wall wart", the microphone-shaped remote control, and of course the bust itself. Basic test includes putting it in "Alive" mode and making sure the head roll and left/right tracking are working. If you press the "Alive" button twice on the remote, the bot will go into "sleep" mode (centers head and eyes and then stops moving). If the left/right tracking does not work, or you hear clicking/grinding noises, fear not (yet). Often when you cut the head skin loose from the body skin, the resistance for head movement gets reduced and the left/right tracking may work perfectly. Also note that the microphone-shaped remote controller is NOT a microphone, just a IR remote to change modes and adjust volume. It is needed to put the bot into "Alive mode" and "Sleep" mode. After this hack, the volume feature on the remote control will no longer work since we are going to disable all the original internal audio. The original manual can be referenced at hightechscience.org - Elvis_Manual.pdf.