Quote Originally Posted by dragonrage
From what i have seen the candidates are chosen by the holy grail itself. Usually a person who is a master or whose family are or were magicians.
Yeah. This seems likely. It also seems likely to me, after the things Caster tried, with the existence of Gilgamesh, and lastly with Lancer's revealed master, that there really aren't any rules, asides from physical ones, to this game. If there appeared to be some rules, they were just imagination of the players, as if they had tried to make it more sensible for themselves.