How do GM choose spells

[The ineffible GM]

I use a very simple method that is based upon two pieces of information.

#1 = Wizards can learn spells from a variety of sources, so it is possible for NPCs to have studied spells above their level or have a larger number of spells at their disposal than is necessarily listed in the book, as I can assume that the NPC has likely been a wizard for at least a little while prior to encountering the PCs.

#2 = The players can't see my notes, and therefor cannot tell if I'm referring to a pre-arranged spell list or just pullnig them out of my rear as we play and LYING about a pre-arranged spell list.


seriously though,
I often decide upon a theme for a character's spells prior to a game, and then just pull out spells as the encounter continues that I feel would suit the NPC's personality, his theme, and the current situation. On occasion, I'll decide upon a couple of spells prior to the game that might be necessary for plot points (i.e. in a BTS game I had a circle of wizards working against the player's P.I. agency, and so when a wizard attacked the agency and then wanted to make his getaway, I used the Electrical Disruption spell to disable the player's car as he drove away).

[KillWatch]

I try to flesh out my NPC mages. It helps me deal with how they will act and react. If I know who they are I know what they will and won't use, what kind of skills they would take and how they would use them. Are they secretive? paranoid? Battle hungry? Ascertive, Crazy? Methodical? Megalomanic? Depressed? etc. Are they interested in basic study of magic and it's energies? to they have a goal and motivation that would shape their choices? all kinds of stuff. OR you could just picka bunch of spells that will reign death upon your players as per normal