@ Wimwick
The consequences? I knew i forgot something. good point.
And it’s true: most memorable NPCs from a campaign aren’t usually the ones the DM thought would be. So when making NPCs everyone keep that in mind.

@ Dungeon
Wow, that’s some great feedback you’ve added on! Thanks, that’s what I like to see from the community.

I broke down the categories of NPCs this way deliberately to make the article easier to write and read. There will often be times where the PCs need to talk with the common citizens to gain information. This can be handled in two ways: 1 – Use of the streetwise skill. 2 – Some simple role playing where you as the DM already know the information that the PCs can learn. I find it’s often difficult to determine exactly what the PCs will do. Therefore, I don’t come up with detailed descriptions of the average NPC ahead of time. I can do that on the fly quickly enough as long as I know the basic background for the area they are in.

You raise some interesting points like how powerful is the NPC. Could the PCs kill the average king on a throne, absolutely. But what are the consequences of doing that? How will the kingdom view them? How will the guards react, how will the royal wizard react? Are the PCs doing this to become the new rulers? Though a king might die in a single hit, I would imagine that if the kingdom was powerful there would be magic that was protecting him and a resurrection spell could easily bring him back from the dead.

@ Rook
I find the most memorable NPCs from a campaign aren’t usually the ones the DM thought would be remembered.

1. Why is the NPC here, what is his/her purpose?

Most DM’s figure this out first. “Hey! Let’s make a cool pirate.” “Umm… why?”
Of all things, we should find out why this character is here. “For example, the town guard is made up of volunteers who take their job rather seriously. While they aren’t professional soldiers, their sheer numbers and determination makes outlaws think twice about attacking the village.” this brings up a good lay out for an NPC, a guard.
But in this article they divide the people into “common citizens,” and “major players.” sometimes from the player’s perspective this fine line is blurred. Some PCs just want to talk to some random people, and that’s okay. But make sure you (as the DM) know why the NPC is there.

2. is this NPC good or evil, and does s/he help or hinder the players?

To some, this is a straight forward. That villain is going to “attempt” to kill the PCs. That guard is going to help fight off the goblins with the players. But what about that villager watching the fight, does he have the guts to help the PCs or will he run, or better yet, is he in cahoots with the villain? Some characters can be complex. For example, maybe that villager is secretly the “masked robe,” who helps those that are in need, even the villains.
It is important to figure out if the NPC can be useful to the players or will the NPC hinder them.

3. How does the NPC look?

At this point it is nice to just give a description, “before you is a humanoid, he wears scarlet robes and a white mask beneath the hood.” While this is good, you must also remember … did I already use that description on someone else? Did I use that same description earlier when they first saw the NPC?
If you have, do not feel bad. We all do that once in awhile. It is okay to be consistent, however, if it is a different character than something should be different. You don’t want the PCs to think that guard is the same as the robed figure. As a plus, DMs should give more thought into descriptions. Maybe the NPC is smelly, or crippled, or shivers at times. These are traits. To make that one villager different than the others, even only a tiny bit (he smells funny) will allow him to stick out for the players to recognize.

4. How strong is the NPC?

“Hey guys, that Imperial king is really old and looks weak, we should kill him and take his gold!”
“Umm… no, he is much too powerful.” “How do you know?”
Can the PCs really kill that one guy if they feel like it? Even though he is a major character? After all the above stuff is done, usually it is time to determine statistics. Now I must give you some personal advice… DON’T GET ATTACHED TO YOUR NPCS!! You can get attached, but keep in mind, that you should be fair and allow the NPC to be mortal (unless s/he is not). Figure out what the NPC has been through and you will get a rough idea of how strong they should be. For the most part, you can figure out HP, AC, Skills, Equipment, basic stats (STR, DEX, CON, INT, WIS, CHA), and money and other items.
Usually a warrior king should be harder to kill than a political king that just rose to power because his uncle died.
The rest… is frosting on the cake. You can make up other personal things with the NPC, such as close friends and etc. The above steps are not complete and I may have missed a few things, but i hope you enjoy it.
I enjoyed the article.