That's a really boring way of saying it.
In any case, there are specific times that I'm supposed to introduce NPC interaction if I can.
But I'm already thinking of a hundred things at the table; so not having to be creative and thematically consistent off the top of my head is great.
Hence this procedure.
What the NPC wants is your agenda for the scene. It's tricky: the player initiated the scene, but it's not really about the PC; it's really just a way to get you to get them to interact with NPCs in a way that actually has a point and might be interesting.
Use reaction rolls¡ to determine how NPC will react if you don't know. Will a heated situation escalate? Reaction roll. Again, only use when you legitimately are at a loss. On a "wary" result (see below), CUT. When it's clear the NPC will or won't get what he wants, CUT.
The object of the game for you in these scenes is to play the NPC realistically trying to get what she wants.
Advanced technique: instead of using an NPC, have another player perform the same function with his PC.
Also: this is actually a lower-order process. You don't just randomly have random NPCs wanting random things from the PCs. This tech should be used as part of a Plot, which I'll have to write up later.
FRAME THE SCENE
For each, have a default available. Don't waste time getting it just so. Ask the question, and, if the player doesn't know or care, go with your default.
* ask where PC is
* ask what PC is doing
* ask who else if anyone is there
* (optional) ask what PC is thinking about
INTRODUCE THE PC
* was there the whole time somehow
* enters/stays without permission
* requests permission to enter/stay
WHAT THE NPC WANTS RIGHT NOW
"You" = the PC(s)
* agreement about something (religion, philosophy, ethical judgment of something someone did)
* get you to agree to come to service/rally/concert/lunch/date
* get you to actually come to event ("Well? You ready to head out?")
* what you really think
* a favor
* get you to ask them about/for/to do something
* provoke you to anger/jealousy/lust/vengeance
HOW TO GO ABOUT GETTING IT
* just straight up, explicitly
* talking in circles around it
* using euphemisms and code
* easing into the topic with small talk or a decoy topic
* fast talk, assuming one way or the other
* suddenly switching to the point at issue
HOW TO END IT
* display of affection
* walk away bleeding
* on the floor
* storming/skipping/slinking/fading out
* remaining quietly, stewing/enjoying
* insults and throwing things
* threatening consequences
* promising something nice
¡ For reaction rolls, I do a Cha check (no 1/2 level bonuses if you're playing 4e). For stat ranges of -3 to +3 (like Moldvay), use 2d6 (6- negative / 7-9 wary / 10+ positive). For -5 to +5 ranges, I use 1d20 (9- negative / 10-17 wary / 18+ positive).