If you experiment, you'll find that a variety of words--including most emotions--are accompanied by special behavior as your Mii talks. The camera might also zoom in to highlight the expression, so it's worth experimenting. Write a post including the word "cat" and your Mii's face will briefly flash a feline expression. Mention Nintendo and its hardware and you'll hear the sound of Mario snagging a coin in the classic Super Mario Bros. games.