|Demonologist by Jon Kaufman, for the NOD Companion|
When I'm writing NOD or Hexcrawls, I usually do not write down the prepared spells for the various spellcasters. There are a few reasons for this, but the top two are (1) it often takes up too much room and (2) it makes some people who run games feel locked in with those spells. Tonight, while writing character stat blocks for the Virgin Woode (progress continues a bit more slowly than I'd like), a notion occurred to me for allowing on-the-fly spell preparation for game masters.
The basic idea is this: The GM picks a spell they want the caster to have. They roll 1d20. If they roll under the caster's Intelligence score (you can substitute Wisdom for clerics and druids, if you prefer), then it means the caster had the wherewithal to prepare that particular spell for that particular day. Easy as pie.
So - I have a 4th level NPC magic-user with a 14 intelligence. He gets into a scuffle with the player characters and I decide I want him to cast magic missile. I roll the d20, get a 13, and that means he prepared magic missile that day. If he wants to cast it again, I need to roll again to see if he was smart enough to prepare it twice.
Now - you might be saying, "Yes, but what if I have an evil cleric NPC and it turns out she didn't prepare a basic spell like cure light wounds or inflict light wounds. That's stupid."
To get around this, it probably makes sense to scribble down a few spells, especially low level spells, that we don't have to roll for to check, at least for clerics and druids, who have all the spells at their disposal. For magic-user, it's arguable that the only spell they certainly have in the spellbooks is read magic.
So - 3 basic spells (using spells from Blood & Treasure - your system may vary) of each of the low levels that we can assume a cleric and druid and magic-user would have prepared:
1st level - bless, cure light wounds, protection from evil
2nd level - cure moderate wounds, hold person, silence
3rd level - cure serious wounds, dispel magic, prayer
1st level - cause fear, inflict light wounds, protection from good
2nd level - inflict moderate wounds, hold person, silence
3rd level - animate dead, blindness, dispel magic
1st level - cure light wounds, entangle, produce flame
2nd level - barkskin, delay poison, resist energy
3rd level - dominate animal, poison, wind wall
1st level - detect magic, read magic, sleep
2nd level - detect thoughts, levitate, web
3rd level - dispel magic, fly, suggestion
Of course, if you don't want to go through this exercise, it's always acceptable for the GM to simply decide, on the fly, that the cleric has cure light wounds prepared, and I'm not going to bother rolling for it.