There is nothing in the world like a good ghost story. Tales of haunted places live through out many cultures and even in this modern era, the ghost lives on every year in the box office through movies like “The Conjuring” or this year's offering of “Winchester, the House that Ghosts Built”. Yet what makes a good ghost? I'm not talking about a friendly ghost, but what makes a good ghostly character, one that will terrify yet interest the reader? I think it comes down to a few factors involving set up and design, but the biggest of them is that a ghost story, above all, is a mystery.
What I mean by this is, every ghost story has a simple question behind it: who is the ghost and why are they there? Some stories are the answer to the tale, your standard back story of the ghost in question that is common of the campfire tale. However, you can also tell a story where someone finds a ghost and then discovers all the whos and whys and hows of their condition. Sometimes it may not even be a ghost but a person pretending (the old scooby doo style of story) or maybe it is something else entirely. How many stories have you read where the ghost was actually a demonic creature of some kind? In truth the ghost story can go a variety of directions, it is all based on what the goal of the story is.
What do I mean by goal? Well, in terms of horror stories, there are three major types that you can find generally speaking: the moral allegorical, fantastical, and psychological. The moral allegorical gives you a moral of the story at the end, be it the story of the killer in the backseat telling you to always check the back of your car before you hop into it or the slasher movie that warns of having premarital sex and partying without paying attention to your surroundings or any warning signs. The goal of these stories is to send a message, to give your audience a thought of what they should avoid to do at all costs. The moral allegorical story is a common version for the ghost story: don't mess with the graves of the dead or you shall end up haunted by them is a common trope from legends involving various graves around the United States (Black Agnes) to the movie Poltergeist.
The fantastical horror story is the cosmic horror of H.P. Lovecraft, the strange, the unusual, the out of this world Cenobites or the tales of werewolves at your doorstep. The goal of this style of tale is to push the imagination, see how far you can push the medium until the suspension of disbelief threatens to fall apart. You can see this in more ghostly tales dealing with spirits and demons trying to reach from beyond to possess someone. The ghost reaches out, trying to become the living again no matter what the cost, much in the way the Insidious franchise has its ghosts bend time within the Nether just to be able to reach the human world again along with the humans who can reach within that place as well with fantastical abilities.
Lastly, the psychological horror focuses on the horror of the mind, but in a different way than the fantastical horror can. This is your stories of insanity, your main character losing their mind to what is around them. This is where “The Tell Tale Heart” lives, “The Silence of The Lambs”, and all those stories where you look into that abyss, and the abyss gives a cheery wave of hello. Any story where you realize as your main characters do that they are the ghosts in the story is an immediate thought in my mind, I mean, after all, so many stories rely on the concept that most ghosts don't realize they are dead. The other branch of the psychological ghost story is the one where the ghosts are helping someone become insane, like say The Shining where the hotel's inhabitants help Jack Torrance lose his mind to the point where he tries to kill his family.
Okay so these establish different types of stories but what is the point of the separation? If you intend to write a ghost story, knowing what type of story you are telling can help you come up with the types of ghosts you are dealing with and what their goals are. Do they want to warn someone of something or punish a misdeed? Do they want to be alive again by possessing someone or are they possessing a thing? Do they want to destroy their mentally or do they think they're alive? Motivation of the ghost drives the style of the ghost, how they will act and how they will interact with our world. Note, this is not the end of the list of motivations, there are as many ghostly motivations as there are human ones. Ghosts are after all, just dead humans. Humans who had life and now they lost that precious thing, but the question is, what will they do about it? That is the question and leads into the mystery at the heart of any good ghost story.