User blog comment:Monkey.D.Me/SOUL FAQ (*^*)/@comment-3119203-20120227192252

My theory is that Brook's devil fruit has already acted. Like, all it did was give Brook's soul a second chance, and then it left, leaving Brook a guy without the ability to swim. I don't think that it is really a second life (though it can be interpreted that way), it just allows his soul free realm in the realm of the living after he has died. Being a soul, it therefore has the ability to do all sorts of things that a soul trapped in a human body cannot (freezing, transportation, etc.).

Now this is a rough draft theory, and is probably full of holes, but if this is true, then possession is completely possible. I think that he is only able to do it if the person he is possessing is unconscious though, or his will might get overwhelmed. I think that this is basically what he did to his own body, (which he returned to simply because he was familiar with it) and to his sword, hence the chilling powers.

Yes, he can die. I think if he possessed a person, and that person was killed (or his own body) then he would also die. I also believe he can communicate with other souls (didn't he explicitly state that he could touch the souls of people with music for power?).

Now we go to the devil fruit issue. Since I believe it has already acted, then he would not be affected by seastone or water. Does he die every time he falls into the ocean then? And does he come back to life every time? I guess so, depending on what you think of the nature of his fruit is.