The apparition of someone who you knew very well has a resonance with me. Twelve years ago my business partner who I had known for nearly thirty years died from lung cancer so I had to buy his half of the business from his widow and made several rearrangements. For several years I had a recurring dream where he had come back, seemingly from the dead, and I was annoyed that all my changes would have been in vain and that I would have to share the business with him again. Because I knew him well and knew all his mannerisms I could talk to him and he would respond as I expected. He was very real. Gradually I learned to adjust, firstly by arguing that I had seen his death certificate, then later that I had bought out his share anyway. Gradually the dreams receded until they stopped but for the few years that they continued they were very real and disturbing. When you have a model in your head of how someone behaves your mind can recreate them in a very realistic fashion and it can be very unsettling.