According the book, this is how not to do it. Mr. White suggests that because everything in nature moves in arcs, the vertical position of the head moves down, then back up (hold your chin and turn your head, and it will be more clear). The books says a straight turn looks too mechanical, and I agree, judging from the poor fellow above. Mr. White also suggests that the features will appear to slide across the face, but I think that's a 2D thing (I wonder if I would have gotten that effect if I had used the Toon shader?). Other suggestions for a more natural feeling character were to add a quick blink mid-turn, and to have the eyes lead the motion (and again, Mr. White says the pupil should move in arcs, not a straight turn).