I've been obsessed with NPCs for a while now. I don't like the randomly generated ones. I have some mods that help but for just editing them in CAS, just use manage households to edit them but make sure (as was mentioned above) that you put them back into Unplayed households. When you edit them they automatically go into played households. Just click on the heart and they will go back to unplayed households. If you don't do this any NPCs will lose their jobs.
I have some mods for assigning roles and preventing random NPCs but for some reason Librarians can't be assigned. They have to be generated by the game so I edit them all the time. There will also be two librarians for every library so if you edit one and see another, it may just be the other's shift.
Good luck!
