GMA World

NPCs as monsters can be inspirational when writing an adventure. They can transform yet-another-room into a springboard for interactions that last for years of game play. An NPC can be much more flexible and interesting than almost any monster, unless that monster is particularly intelligent.

The NPC can also be butchered withing seconds of encountering a party, or can reside in the limbo of "rooms we've never explored" until the DM has forgotten whatever reason they had invented for them being there.

A good way to define NPCs in a shorthand which doesn't invest too much time (which might be wasted) while capturing that on-the-fly inspiration that often strike while placing encounters is: GMA.

G is for "Goal"

This is what the NPC is currently trying to achieve when encountered.

M is for "Motivation"

This is the second-order goal: why are they striving for the stated goal?

A is for alignment

This is what means they will use to get to their goal. A chaotic evil character will clearly have very different methods than a lawful good one, even if their goals are the same (their motivations might be different too, of course).

Examples:

Where's Princess?

Goal: Finding the Princess

A cliched situation - the party meet a character in the dungeon who's goal is locating for a missing princess.

1. Dame Joanne, 9th level paladin. The princess poisoned her own uncle and fled into the dungeons. Dame Joanne's motivation is that she has been charged with bringing the princess to justice. Her alignment is, of course, LG and so her method will generally avoid simply beating the princess into unconsciousness and carrying her out. However, it may come to that if the princess is too reluctant about being taken back.
2. Magpurse the bounty hunter, 9th level fighter. Motivation is simply that there's a reward for the princess: 1000gp, alive or dead. Magpurse is CN and is unlikely to simply kill the princess and carry her out, but it might come to that if the princess is too reluctant about being taken back.
3. Olfish the bounty hunter, 10th level assassin. Motivation is the same as Magpurse, but alignment is LE. Ofish is more likely to join up with the party and manipulate them into helping him find the "escaped assassin", then if possible abandon them somewhere dangerous while he kills the princess, cutting her head off in order to claim the reward without having to haul her body out. He has a box for this purpose.
4. Father Darney, 10th level cleric. Motivation is that the princess is his daughter and he wants to help her, but he's LG and needs her to come back and face the court.
5. Quick Tim, 11th level thief. Princess's secret lover. Chaotic good with no intention of returning her to face the court, he's got plenty of cash and they can start a new life somewhere else.

These five characters share the same goal but their motivations and alignments mean that the will interact with the party (especially a party that has already met the princess) in completely different ways, but your notes only need a line or two for each one.

 Goal: Treasure hunting

1. Billy Slim, 2nd level thief. Motivation is simply greed. Alignment CN - finder's keepers.
    
2. Dunhelm, 1st level cleric. Famine is abroad in his land and he thinks there might be money to be found here to pay the high price of grain in the city market. LG - willing to work with others but will try to take any share in cash. Generally keen to leave once his share is 500gp+.
    
3. Blansom, 1st level cleric. Famine is abroad in his land and he thinks there might be money to be found here to corner the market in grain and sell it to the hungry rich for a profit; he might even just destroy it all. NE - will avoid any party that seems Good aligned, will otherwise try to join up for the protection value; if you can't look after what you have then it's fair game to him.
    
4. Kimson, 2nd level monk. Seeking an scroll stolen from his monastery by bandits he believes are in this location somewhere. LN - will join almost any party who agree to let him have the scroll if it is found; will leave if he decides their word is worthless.

 Goal: Finding the Lost Temple of Set

Gold and  Silver Jewelry

1. Tut-Aton, 7th level cleric. Motivation is to destroy the temple, breaking a gateway through which Minions of Set can be summoned. LG - will not tolerate evil characters at all and will try to destroy them where possible.
2. Tut-Mose-Akon, 7th level cleric. Motivation is to secure the gateway and summon Minions of Set to become the core of his army of conquest. CE - delights in human sacrifice (and is looking for some victims to open the gate) and general havoc.
    
3. Tjanefer, 6th level paladin devoted to Horus. Motivation is to find the temple and ambush Tut-Mose-Akon, killing him for his many crimes. LG - worried about the urgency of his mission, Tjanefer will tolerate non-Good characters who agree to help him. Will not be diverted but does mention, if needed, that the temple is said to have riches within.
    
4. Hermione, Sage devoted to Hermes. Motivation is research into the activities of the ancient order of Set Unbound. N - will join up with any reasonable-looking party that she thinks won't betray her. Will spin tales of lost gold and gems, but personally interested in inscriptions and regalia.
    
5. Panaeus, 9th level druid. Seeking aid against a mage devoted to Horus who is expanding his domain. N - will try to befriend any strong-looking party and eventually recruit them to his cause. 

Motivations need not come from within - a character may be doing something because their family is being held hostage or because they have been charmed, in which case the true motivation belongs to the person who is doing that to them.