And the way Eliezer seems to have put it sometimes, where one glance at a line of text will change your mind?

Going with the "dead loved one" idea mentioned above, the AI says a line that only the Gatekeeper's dead child/spouse would say. That gets them to pause sufficiently in sheer surprise for it to keep talking. Very soon the Gatekeeper becomes emotionally dependent on it, and can't bear the thought of destroying it, as it can simulate the dearly departed with such accuracy; must keep reading.