This is a very handy piece of knowledge. The profession of any NPC who is walking around, with the exception of peasants and certain plot figures like Mr. Malwick, is undetermined until you angage in conversation or right-click on him or her. Then it is fixed until you leave the map, at which point it becomes undetermined again - unless, of course, you hired that particular NPC. If you did, you won't find another in the same place later, not even after you sacked your NPC, so there is a danger of running out of potential hirelings until the area respawns.

Certain professions are off-limits in certain area's. The most important restrictions are, if memory serves: