I think the easiest way to explain it might be as follows.
Your soul is who you are - but you are not necesarily ALL of your soul. The person who is alive at the moment is a portion of the soul and that soul could have a large number of previous lives. When you are alive there is a "wall" up between the currently alive personality and former personalities that make up the greater soul. Any crack in that wall could allow opinions, memories, or beliefs of the main Soul to influence the current personality.
The alive person is part of a greater whole - their Soul. They are independant of it, but it can still influence them through various means (dreams, feelings, sense of right/wrong etc). That's the way I've taken what OE has given us so far. One major factor being the dissonance/resonance between the current personality and the former lives. I imagine the Paladin whose former lives were full of vice and injustices might have a serious psychological issue from what he needs to do and from the guilt of what he believes he has done previously (or how he plans to "attone" for it), just as an example.