In my campaign, it depends on the PrC, but in general it will involve some roleplaying, which is why I've asked my players to let me know 2 levels earlier if they plan to go for a specific PrC. Also, the roleplaying will sometimes involve just the PC and sometimes it will involve interaction with, and perhaps training by, specific NPCs.
For example, the Peloran cleric who recently joined the party did so because of a vision from Pelor. I've discussed the possibility of him heading for Radiant Servant of Pelor, and in his case there will be some in-game justifications, but overall his assumption of the PrC will involve individual roleplaying and not NPC interaction. But if the Ftr/Rog were to head for the assassin PrC (yes, there are non-evil assassins IMC), he would probably have some interaction and training/testing with an assassin guild.