Roleplaying isn't something that magically makes your game fun. What it does do is allow you to have a little fun and play a game within your game. Essentially you use the EB mod to make your own game with your own characters and stuff.

If you have an Indolent, lazy, crappy King/leader then you might want to have him sit in the capital and call people back to the centre of your empire and maybe not worry about countering threats to the empire, even if you lose a few towns. This is very much a type of reactive role-playing where you just do whatever the traits tell you (and hope your bad king dies soon so you can go back on the offensive). Maybe even do it with multiple characters, such as having your crappy king recalling troops to the home provinces while you bright talented general single handedly conquers Iberia with a skirmisher unit and half a roll of gaffer tape.

Another way is to have a more proactive roleplay angle. YOU are the king and you make your own little story that goes along with how you want it to happen. Can be quite fun especially with the use of console commands (cheats) to add/subtract money, move armies, create armies. Force diplomacy is almost a must with this style if you want something complex to form. Some AAR's make full use of this style. If you really want to go over the top you could try self modding bits and renaming, recreating alternate factions. I think Maion killed the Casse and made himself a shadow faction instead.

In the end its about doing whatever you want with the program that has been provided for you, winning and hoarding territory and slamming the AI is not the only goal anymore.