Adding to what Verteiron said, the SetGhost option makes it so the actor has no collision - so they can walk through walls, etc, and also means they can't get hit. It makes absolutely no change to their appearance.
I tried that and couldn't get it to work. My ghosts collided with all sorts of things.
I think ... how do you script a ghost to walk through a wall anyway? I mean what about navmesh, for instance?