A summary of further serious/usable responses:
Centaurs -- Always talk in terms of astrology. View any suggestion of riding them as deserving death. Eat both horse and human food for their two stomachs.
Medusa -- Hire models to "pose for a statue", keep cages of mice to feed their hair, decorate their lair with real statues as well as victims.
Ettins -- Argue between heads. Play "good cop, bad cop" (if they parlay at all). Heads fight over food like two dogs.
Ratlings (Nezumi, Skaven, etc.) -- Always sniffing, especially in a new place. Gnaw on wooden objects (spear shafts, staves, etc.) when bored. Attracted to glittering objects.
Yuan-Ti -- Taste air with tongue. Stare -- without blinking -- at people. Bob their heads.
Troglodytes -- Move around a lot when angry or frightened; it helps spread the stench from their musk. If parlaying, attribute the most authority to the worst-smelling member of a group. Change skin color in order to express feelings.