If you need an NPC quickly here are a couple of ideas.
Do you need a classed NPC?
Use one of your own old characters from back in the day. This works best if you have played a character of the type you need. However, you can easily transfer names and traits onto any class.
You can also steal a classed NPC from another GM’s world. You may not know their stats, but you know how to present them.
Do you need a generic NPC?
Steal one from one of your GM’s campaigns. Tavern owner, shoop keeper, etc.
Do you need a trait for a monster?
Take examples of how the past GMs you have played with expressed their orcs, ogres, giants, etc.
The beauty of taking characters from one game to another is also that it doesn’t have to be rule specific or genre specific. For example, a greedy merchant can be a robber baron, Wall Street investment banker, space corporation executive, etc. The main thing you are after is the portrayal of the character, the class, race, and abilities are secondary.
The key to acquiring new NPC concepts is to pay attention to the repertoire of other GMs. Playing in other games whether in person, online, or at conventions, is a great way to get exposed to NPC “templates.” The more memorable the portrayal, the easier to recall. However, not all memorable NPC’s are over the top, larger than life personas. Many are regular people, and can be bland or generic. Reviewing lists of character traits in the AD&D Dungeon Master’s Guide is a good way to be on the right wavelength to pull out a believable persona.
Friends, family, wait staff, work colleagues, basically anyone you have ever interacted with can have something to offer to help build NPCs.
Movies, TV, books, magazines, etc. all have characters who have traits that are memorable. All of them are fair game to help you make the next town’s tavern owner different from the last. Street urchins can have traits and manners of speaking like the characters from The Little Rascals (I prefer the originals from the B&W shorts).
If you struggle with on the spot development of NPCs, build templates that emphasize a major trait but flavored with minor traits. For example, a greedy merchant template could take the used car salesman trope and flavor it base on your own experience buying used cars. Some talk fast, some dress gaudily, others dress sharply, some dress slovenly. Greed can take different forms. One is honest so does not lie to gain the sale, while others have no scruples other than whatever it takes to get the sale.
You can also sit with the DMG and roll on the tables to build random NPCs. They can just be collections of traits on index cards, and you just grab the next one when it is needed. You can also script them in a spreadsheet, power shell, bash, or programs like Inspiration Pad Pro. Then you can generate hundreds and pull one out when you need it, or even on the fly at the table.
Find a way that works best for you. Build a mental catalog as you go on a long walk or drive, or mow the lawn. Create a card catalog or lists on the computer. Whether you speak in different voices, or just describe their mannerisms and tone, you should always be able to come up with a new NPC’s characteristics on the fly.