Creating NPCs

Minor

No matter whether you're creating a main character, a side character, or just a one-off, improvised character, you'll always need the basic characteristics. Their appearance, their mannerisms, their way of speaking to the players' characters, they way they interact with the players, and so on.
With this alone you'll often have a pretty decent beginning. Within these elements there can be a lot of story driven elements too. A limp may indicate a fresh injury, a shaky voice my indicate nerves, and a pushy demeanor may be a clue this NPC is either desperate or eager to get the players to do or believe something.

If there's a possibility for this character to be part of a combat encounter, you'll also want to sort out all their attributes and abilities as well. If you create this character out of the blue, write down the attributes you made up so you can use them again later, assuming this character survived the encounter.

If you want to go into more details, you could hide details within these minor traits. The character might wear a religious pendant, wield a specific kind of staff, alter their appearance with magic, or hide another detail in plain sight.
Whether you want to go in more details or not, if a character is supposed to be prominent or memorable, try to focus on specific aspects of them, especially the ones that stand out. Doing so will make them memorable in and of itself, as this helps to keep characters different from each other, and prevents giving the players too much information they might wish to remember.

I tell you what I want

Right, time for perhaps the most crucial part: figuring out what the character wants. Knowing what a character wants means knowing what drives them, what motivates them, and thus how they interact with the world and the players' characters.
I've purposely used the word character so far, rather than constantly referring to them as NPCs. They are characters first and foremost, the fact they're not playable by the players is irrelevant. So, treat them like actual characters, actual beings with wishes and desires. It doesn't matter what the character is either, they're all beings to some extent.

A "simple" wolf NPC may seem like nothing more than a filler encounter, but it works way better if you use its motivation in a story setting. A wolf may wish to defend its territory, hunt for food, or protect its babies. A bandit may love to steal, need to feed their children, or be pressured into stealing by a stronger force.
The characters don't even have to be "living", a simple robot still has something that drives it, even if those are commands. A sentient island will have something that drives it, even if they're merely instincts to keep itself afloat and away from hostilities.

What a character wants will also lead to their other important attributes. What they fear, how they go about getting what they want, who they're willing to ally with, how they'll treat the players' characters, and so on. So once you've figured out what drives a character, role playing them will be much easier.
Role play, and more specifically the play part, is the crucial element here though. Playing your characters will drive the game forward far better than trying to play set pieces. Have your world, and thus the characters, react to your players' characters, rather than just your players to your world.

Other attributes

Lastly there are the other attributes, the ones that help flesh out a character even further should there be a need for it. Their alliances, their relationships, their moral codes, their tastes, and so on. These elements usually aren't all that important to most characters, but when a character is a major part of the campaign, these can be worth writing down.
These elements, too, offer chances to show a bigger part of the story universe or campaign. Certain characters or creatures might avoid eating certain foods, walking in certain areas or lights, avoiding specific races, and so on.

To give you an example, let's say we have a stereotypical baron who happens to be a vampire. What are his minor attributes? He's charming, tall, fair skinned, black hair, well spoken, well mannered, rich, and seemingly loves to throw parties at his mansion.
What does he want? He wants his next victim, preferably a young maiden. He tries to find them during the parties he throws, while at the same time pretending to be a regular human with a heart of gold. So, he also wants to avoid being discovered as a vampire.
What are his other attributes? He obviously hates garlic, so his food never contains any. He avoids daylight, he smiles with closed lips so his teeth don't show, he is very flirtatious (to seduce his next victim), and for some odd reason there are no mirrors in house house. How strange.

As you can see, the character is pretty detailed already, and everything is centered around the two things he wants. You can add more details after this as part of the other attributes section, details that aren't part of what the character wants, but they're not always necessary, and they can often be improvised on the fly. Details like his favorite wine, the type of music he listens to and/or plays during his parties, his romantic types, and so on. But you can also flesh the character out with more character defining details as well. How did he become a vampire? What happened to his family, or does he still have one? How often does he move places to avoid people figuring out he's immortal? And so on.

We're probably treading on novel writing territory here, as most of such details aren't usually discovered in an RPG campaign, but sometimes you do run into a character who sticks around for a long time, and the players might keep asking questions, so it can be worthwhile to think about this in between gaming sessions.
If you are interested in a more in depth character creation process, check out the character creation guide part of the writing & other guides on this site.

Final notes

If you're still struggling with creating a character, try to focus on what you'll definitely need from that character. Do they have to oppose the party? Do they have to be able to answer a lot of questions about the world or the party's mission? Do they need to be influential? And so on. Then answer them accordingly with questions like "Why?", "How?", and "When?".

Focusing on these elements will mean you'll at least cover the things you absolutely need, anything else can be filled in later. Make sure you take notes when you do fill in important elements on the fly, that way you'll both remember them, and be able to interweave them into the character more easily.
"Why?" is always a good question to keep asking. Why does this character oppose the party? Because the party wants to stop them from ruling the nation. Why? Because his rule comes at a huge loss of live. Why? Because two different species will fight each other because of it. Why? Because he's the leader of a vicious, narrow-minded species who seek to destroy all others. And so on.

Lastly, if you prefer a checklist of important characteristics you might want to fill in for a character, consider the two below. The more detailed you need your character to be, the more points you'll want to cover.

Basics
- Name
- Sex
- Occupation
- Appearance
- Unique feature(s)
- Mannerisms
- Location

Details
- Desires
- Secrets
- Strengths
- Fears
- Allies
- Enemies
- Current goal
- How are they part of the world