I don't know if this will help anybody, but I was having this issue with NPCs and it turned out it was because of the Unique Character mod and the Expressive Facial Animations mods - I didn't think that would be it, but after putting the malehead.tri and femalehead.tri from expressive facial animations into each race/gender folder in Unique Character the problem is solved. I really don't get why Unique Character apparently affects NPCs, but this was driving me crazy so maybe it will help someone else!