When it comes to expressions the mouth kind of accents the eyes. It helps you to fully express what the eyes are trying to say. Like eyes, mouths can range from very simple(a simple line) to complex. It is a little easier to figure out what you need for mouths than it is for eyes. Everybody knows a smile from a frown. But a smile can be smug. A smile can be sarcastic. What is different between a mouth that is talking and a mouth that is laughing? What is the difference between a mouth that is talking versus a mouth that is yelling? Again there are no set rules. But I will usualy take a mouth that is talking and add teeth to get a mouth that is laughing. Or I'll take the same mouth and add a tounge to get a mouth that is yelling.
Like eyes and noses I like to draw the mouth a little to the side and paste it when I'm done.
Our character is yelling. But, I didn't give him a tounge. Can you guess why? It is because he is a mad scientist. I want him yelling intelligently, insane yes, but not a ranting rage.
All this guy needs now are ears and some hair. Okay, maybe he needs a little professional help as well.