I have so rarely been on a flight with any empty seats that I can't even imagine asking, but on the LHR-YYZ, a woman in C put her purse in the foot space of B and I (in A) thought "that's interesting". It turned out B remained empty. Also the FA had protracted conversations with her every time she passed, to the point of forgetting to serve me the beverage I'd ordered. I inferred from all my observations that my row-mate was off-duty crew and knew how to work the system. As a civilian, I don't think I'd have a prayer.
I don't think there is "a conversation" that makes it happen; I think you have to be a certain person or category of person to have that conversation.