Making faces for NPCs is incredibly fun. Not only do I get to practice my art (and goodness knows I could use practice when it comes to character portraits), but I get to give them all kind of wacky emotions. I can go for a subtly smug look, or a look of utter boredom. Or maybe even insane grin or two.
It also helps quite a bit with dialogue. As I'm drawing a character with dialogue that's ready, I can really play up the emotions. Or if I'm drawing the character first, their look gives me ideas about what kind of personality they may have. It's a pretty fun process.
I'm definitely glad I decided to make NPC faces, too. :)