Don't mean to come across as a know-it-all on this topic but I've had my own experiences and thought I'd share some of my insights on that here....
It could be one of four things:
1) The person’s pastlife or pastlives
2) A human discarnate’s most recent incarnation on this plane.
3) A non-human intelligence that has taken over a discarded shell of a human discarnate and is posing as that person. (Rudolph Steiner talked extensively about this one in his lectures)
4) The thought injections from a higher intelligence for any number of reasons.
In my view, I think we’re really treading into murky waters when trying to discern which one it is but consider a good many uninvited temporary #2 takeovers are for them to live through a living host to satisfy an unresolved spiritual, emotional or physical addiction. The human host would be one who also shares their addictions. For #2, I would think that there’s absolutely zero interest on their part for them to provide information about their previous incarnation on the earth plane in as much detail as we are seeing in pastlife experiences.
But for #3, their purpose is to wreak havoc and inflict addictions and other negative impulses onto the host. But nevertheless, both #2 and #3 ultimately seek to attach and control the host and there’s a well-documented process involved in that.
Neither one of them want you to know who they are and only reveal themselves in special circumstances. But I won’t go into that at this time because it would take a book to cover that topic except to say that #3 may be more apt than #2 to provide the host with a pastlife narrative in order to have the host bond more with them. They are much more clever and devious than #2.
So that leaves us with numbers 1 and 4.