1. They can't kill you. They can't pick up a knife and stab you through the heart. They aren't that strong. If they can move a knife, it is to float a small one a small distance. Too much power is required to actually stab someone. They can scratch the flesh, they can chill you, they can frighten you by appearing suddenly if you're already scared. I guess they could give you a heart attack if you were about to have one anyway, scare you to death, but they can't physically harm you more than the scratch marks.
2. Ghosts can't travel. They can move from one room to another, one side of the house or yard or area (like a farm or mansion or plantation) but they can't get in your car with you and follow you back to California from Jersey. They also can't be in two places at once. Two ghosts, two different places.
3. Ghosts can only be in the place they died OR someplace that has something special of theirs they cling to. Say, a bed or a vase that belonged to them...they might be able to follow it after a move, but it would take a very strong bond with that object to allow the ghost to go with it anyplace other than where it belonged when they as live beings owned it. (This does not include the vibes a psychometrist can pick up from an object. Those are the gift of the person, not the spirit of the object.)
4. Ghostly apparitions can have attitudes. If the person was nasty when alive, he or she will more than likely be nasty as a spirit. If they were nice, they'd have passed on after death to someplace else, not condemned to walk the earth for some reason. Remember, the good die young and don't linger here. (I'm not sure where they go, but it isn't here. I certainly hope it is someplace better.)
5. Ghosts cannot appear to be solid. You can see through them. This is something movies got right. They may not even have full figures, just outlines on the bottom, since they really only try to materialize their faces and upper bodies to get your attention. Every bit of energy they get, they're taking from the viewer, so it is natural to feel drained after seeing an apparition. They've used part of you to show themselves.
6. Poltergeists are not really ghosts. They are energies transferred by powerful exceptional children, usually only up to their mid-teens that make objects fly around the room or hit the floor or whiz past your head. Kids don't know their power. It is telekenesis to the max. Children with this ability probably don't know they have it, but get them upset and they produce the energy necessary to fling ashtrays, books, figurines, etc. across a room. Even if they're not in the room.
But they're not ghosts. Geist in German means spirit, but not ghost, more like a fervor, a passion, strong feelings.
7. Let's face it. Ghosts can't eat or drink. They might be able to hold a glass if they manifested a hand, but it would take an incredible amount of energy. Where is that energy going to come from? The living person standing right there or somewhere nearby.
8. Hollywood has done great things with ghosts. So has television. So have novels that have given ghosts additional powers and abilities. That's why it's fictional. That's why we get shivers over ghost stories and delight in what ghosts are supposed to do. Too bad most of the time, they can't do much of anything but float around, absorbing energy we need ourselves.