who here is a walkin, knows a walkin, or has information about walkins?

i've become very interested in this topic, and would like to know how "common" it is. and any other information some of you wise souls may have regarding walkins.

i know a lot of the basics, and that many people realize they are walk-ins after a death experience, near death experience, or other traumatic event. but does anyone know if this ever happens because souls simply agreed to do so? can it be just a decision made prior to incarnation, or during a meeting on the astral plane? any information anyone has would be VERY helpful! thanks in advance! :)