Don't overcomplicate! I've found keeping your supporting cast NPCs one-dimensional is far easier on the Game Master than taking the time to flesh out an NPC with a detailed personality, description and backstory — most just don't require that much effort, save it for the major players. Over the years I've learned to make my role easier by focusing on one or maybe two things that will make your NPC stand out from the crowd; whether that's a personality quirk; a much loved and always worn garment or just the way their turquoise quiff jauntily flops from side to side as they go about their business.
If you can, prepare a little beforehand: make a couple of notes about the innkeeper in the next village. It's not about spending hours of prep time on these characters, your valuable time is much better spent elsewhere, but a short one or two line note can turn a dull night in the tavern into an unforgettable encounter with Epotimus Bloom — a weatherbeaten travelling merchant; looking far older than his years and sporting a rather oversized and oft patched purple hat with an enormous brim that extends past his shoulders.
Think about things like how the character acts, looks, smells and talks. Does she have a distinct mannerism like always casually twisting her golden locks around her fingers while never looking at you directly in the eyes? Does he have a particular smell like the aroma of aromatic spices that he uses to prepare his famous spiced pies? Do they always wear an unusual garment like a black top hat, even though they are an otherwise scruffy a street urchin?
Should the NPC become a beloved regular often featuring in your player character's lives, then is the time to maybe flesh them out a little bit more between sessions — don't go overboard though unless they're set to become a major player in your world. Maybe our afore mentioned Epotimus Bloom is a regular at the inns and taverns across the county and beyond; with his oxen-drawn rickety cart carrying an ever changing supply of barrels and firkins filled with speciality local ales that he collects from, and sells to, drinking establishments far and wide. Thus we have an NPC that is likely to cross paths with the party throughout their adventurers, and maybe they'll discover a favourite ale to be enjoyed between friends whenever they meet.
Don't be afraid to ham it up and over exaggerate the quirk, and mentioning it every time the player characters cross paths with your NPC will ensure that they're never forgotten. You'll find your players will care far more for a character that they feel that they know, even a little — or maybe dislike that bit more for those of a rather unpleasant demeanour.
Use this to your advantage when creating adventure hooks for your adventurers, who are far more likely to care that the young lad — who always greets them, as they enter the village, eternally cheerfully with an ear-to-ear, buck-toothed grin and perpetually pestering his heroes for tales of their latest escapades — is suddenly absent, having gone missing while reenacting the parties last adventure.
Oh, and before I forget — have a selection names for your NPCs pre-prepared if possible, or a handy random name generator. Every unforgettable cast member needs an equally unforgettable name!
I find it works for me to flesh out the minor NPCs a bit further if I know they're likely to interact with the player characters on a regular basis, like our esteemed Epotimus Bloom, or if the group is probably going to spend a significant time during an adventure in the same locale.
In particular innkeepers and their families and/or employees, and possibly a few notable patrons of the establishment, are a good place to start. As we know many an adventurer finds their way to the local inn or tavern, and a little time spent having a bit of fun creating the characters and personalities will make your role as a Game Master easier and the world more believable.
Make a note of everything! If you've come up with anything on the fly, make a note of it and add it to your records between sessions. A little bit of bookkeeping will pay dividends when your party heads back to the capital, and will be expecting to see old friends at their favourite inn. Maybe those old friends have moved on and the establishment is under new management? but, that's another tale.