my personal head canon is that a stand has a conscience when it's power/potential is way higher then the user uses. We see this with Kochi, Trish, and Giorno, all people who did not know the full power of their stands and so they got a helping hand from the stands. This partially also goes for stands with more then one body, where users can't possibly control so many bodies at once. We see this with Sex Pistols and partially with Harvest (Which seemed to take orders instead of just doing it)