I really can't figure it out. How do I change a character's facial expression or mood, to take creative screenshots?

There was a mod for that in Fallout 3, "Face Aches" i believe? I haven't seen one for NV yet.

Ask MadCat in the Screenshot thread. He often has his character pulling different faces. He likely uses a personally 'ported Groovatron, though I don't really remember.