When you add levels to NPCs to make an encounter more challenging or to compensate for larger parties, how do you assign additional feats and skills? Is it just arbitrary, or is there a system I can use?
Corebook page 450 "create NPCs".
Anyway NPCs gain feats at the same rate as PC (1 every 2 levels after 1st, one more for humans).
Skills are also assigned as for PC classes,the number of skill ranks gained depends on NPC class (6+int for expert/2+int for warrior....),exactly which skills are class skills also depends on NPC class, just like PCs.
edit:for non playable races or creatures/monsters the rules are in a Bestiary 1 appendix
When you add levels to NPCs to make an encounter more challenging or to compensate for larger parties, how do you assign additional feats and skills? Is it just arbitrary, or is there a system I can use?
I do not know of any offical system for adding feats/ skills/ spells etc. to an NPC.
The CRB-(Core Rule Book) has in the chapter on NPC's chapter 14 in my copy a list of suggested feats by the role of the NPC. This might be considered the closest thing to an offical system.
If you have the GameMastery Guide they have a section of pre made NPC's that you can use for insparation.
I start by asking myself what role is the npc to play in the game/ encounter?
A lot of feats/ skills will depend on what their role is intended to be in your game. For example the feats I would pick for a Melee Fighter will differ from the feats for a Ranged Fighter (Fighter in this case refers to the style of combat not the class).
One factor that I consider in NPC creation is am I making a one shot set of stats for one encounter or are they a more developed one for multiable apperances in the campain.
For skills I often look at the "class" skills that are listed for a class as teh starting point for their skill selections they can guide you in what a class should mostly be good in, but remember they are only a guide and a char might have a skill that is not a class skill for any number of reasons.
The CRB-(Core Rule Book) has in the chapter on NPC's chapter 14 in my copy a list of suggested feats by the role of the NPC. This might be considered the closest thing to an offical system.
I'd forgotten about that section. That is actually very helpful. Thank you.