Why assume possession is always a take control thing? Perhaps it could be a piggy-backing voyeuristic thing instead. Since it's all make-believe anyway, one gets to make up the magic rules to suit - exactly as all religious people do. So a non-coercive (while still not necessarily invited!) angel-possession might merely involve experiencing everything the human does but, at most, only be perceived by the host as a general sense of otherness being present - the sort of mental aberration to which humans are prone anyway (eg deja-vu and religion).