It will take a photo of your face and basically you have to throw balls at it to stop it from attacking you. It's some augmented reality stuff where you have to sweep your DS from side to side to "spot" the evil faces (which was mine) and just keep throwing stuff at it.
You only take your photo once but there's lots of "expressions" cos the game will stretch your face to create different expressions! You will look silly, evil, sad, etc. Mad fun!