I try to… of course it’s a sliding scale from random merchant to pivotal NPC who's at the center of the adventure.
Side tangent, besides voice and speech patterns (which I'm not very good at improvising) I found that portrait images really help leaps and bounds in getting important NPCs across and set a tone for them. Most recently did that with photocopied and enlarged portaits of Cigfa, Fuilcwian, Malm and Taim in ye olde Loons of Long Hill scenario for MERP. Worked a treat.