You can't run an interesting D&D campaign without interesting NPCs.
They are the best tool to give to the players the right feeling and mood of your campaign and to make alive the world you have created.
They work much better than long descrptions of places or interesting storylines:
- they are directly in contact with players
- PCs interact with them, with NPCs they are active, not passive (they do not simply 'listen' your description and they're not 'restrained' as by the story)
-with NPCs you as DM can be an active part of your world, you can step into it and flesh it out
You can do many things with NPCs: provide players with informations, services or other stuff, give advices or adventure hooks, or simply create atmosphere.
Whatever you want to create a NPC, just follow these steps:
Don't make a long description. It's useless to write for hours about a NPC's anatomy or appearence or background. Maybe PCs will not talk with him at all. descrizione non deve essere lunga. Write only some basic, important things (such as name and an important characteristic) and go further only if needed. The only real important thing is that PC can remember about him if they'll meet him again.
Example: Mark, guard sergeant. In his 50s with scars on his left hand.
What is its behaviour? How does he talk? Walk? Why he does so and so? Again it's useful to mske PCs remember about him and again you needn't work too much; focus on few things.
Example: sergeant Mark always barks his orders with a loud and heavy voice. Two or three nights a month he gets drunk, alone and in silence, at the local tavern.
Create at least a secret
Every important NPC of your campaign must have a secret behind. There must be something which, when discovered by players, can make them say "oh...Then it was worthy to talk/interact/go with this NPC". More mysteries you give them, more they get interested and more they want.
As usual: Mystery, interest, fun.
Example: Mark is a Fighter who knows many manoveurs unknown to many warriors. Alas, many years ago most of his men were killed in a battle against an undead horde where his battle prowess was unable to save them. He feels guilty for his comrades since then, he drinks to forgot and he has sworn to never use again those manoveurs.
Really, it's one of the least important things. In many cases you can simply use statistics taken from the 5th edition MM, maybe with little differences.
Example: Mark is a veteran with 6 manoveurs of the Battle Master, with 5 superiority dice (d8).
Follow these steps and in few minutes you can create a NPC who will remain in your players' minds.
This is your goal.