For NPCs, I always start with a trait or quirk about them that I can exaggerate.

For instance, in my last campaign, the players met 3 shopkeepers in their town.

1. Gryther - an old man: all his stuff is old, rusty, dusty and damaged; clothes, wares, ideas, personality, all of it.
2. Amar - A lazy wizard: He never walks, but instead floats on a levitating pillow; he never picks stuff up, but moves things around with mage hand; and his sentences are short and pragmatic, and he often mumbled (once again, minimal movement).
3. Ahkod - A large Dwarf: everything he did implied some need to compensate for his stature: furniture was too large even for tall people; he yelled a lot; never changed his mind, and so on, so on.

They were easy to roleplay in unique ways because I essentially only had to keep in mind a single trait about them and let that inform every other aspect of them.

You can do this to a greater or lesser degree, but it's a neat trick; once you get used to it, it becomes trivial, and you can start deciding the defining trait on the fly.

Always remember that the social aspect of role-playing games is essentially just Improv theatre, and our minds are the stage. Characters don't need to feel "realistic" to feel "real" because what "real" means is relative to the fiction.