So far I haven't found people not playing their characters as they wrote them to be a problem. The personality of a character is what makes someone want to roleplay as them. From my recent experience. It could be that when I GM my RP Ascension a part of understanding the world is to have a characters personality run up against the culture of the world. A cheeky thief for example would have to take a liking to areas that are not heavily guarded. This will require a understanding of what parts of the world is vulnerable to thievery. In this way personality meshes will with character creation and world building.