It would have to know that using the character name call with text will tell it to animate the mouth.
This would be used the same way it is now but with the eyes automatically looping all the time and the mouth only showing (and animating)when the text for that character is being typed on screen. In this make-believe example the code is:
Image mii happy = ('characters/mii/happy.png', 'characters/mii/mouth.png', 20, 50, 20, 120.
