You hire NPCs by offering them money. The better they are, the more you will have to offer. Several attempts may be needed before you succeed. The principal use of NPCs is as bailiffs to run your fiefs and lead your army.

At the beginning of the game, the 6,000 NPCs are evenly distributed around the game fiefs. About one percent are "Mighty Men," because they have an exceptional array of skills and characteristics. Many of the Mighty Men are based on actual people of the period, so if you encounter a fellow names Chaucer or Hawkwood, it's no accident. Another ten percent are "wannabees," sort of diluted Mighty Men. Fewer skills, but still excellent people to hire.

Unhired NPCs move randomly during the update.  When an NPC moves during the update, and his random direction would take him into an Off-Map, Sea or Mountain fief, he just does not move and stays where he is.

The principal use of NPCs is as bailiffs to run your fiefs

On the Household Affairs menu you can do an analysis of NPC skills with some restrictions.  You can  only analyze your own NPCs.  No analysis of pregnancy skills and birth skills are allowed as this deals with gynecological matters that were not known back then.  The hire skill rating will look a bit funny than the other skill ratings anything above a nine in most skills is good. Becasue of the way hiring is done, anything above a 1.5 is good with a 2 being excellent.

When an NPC dies, they have a new first name generated and then placement is randomly determined.  Wherever they show up, the language of that fief becomes their new language.