Basically, there are two types of ghosts. Ghosts are spirits of the dead that linger on earth because they cannot get to the afterlife, or they have unfinished business here on this plane. Sometimes their business is known, other times it is not. Family legend may say that the spirit is bound to the earth because it is looking for something, someone, reliving some past misdeed...all carefully written down or passed through family for years and years.

The first type of ghost is the one that can appear at any time, in any place it is bound to. A house, a property...either where it met its end or a place it was particularly fond of. This sort of lets out cemeteries for this type of ghost. If you were going to meet a ghost, this is the one you'd most likely see or hear or feel. They are free to move within their small location and can appear and disappear if they can accumulate enough energy to enable them to do so.
These are the best kind for stories as they can pop in and out of places and scare the garbage out of the person who sees them. They can deliver messages, lead people to discover things, respond (sort of) to questions. Since they really are stuck in this limited space, they are not really aware of the present, only their own past.
To goose them up, to give them awareness of what's happening now, that's stretching facts, but great fiction. Think Ghost and Mrs. Muir here.
He guarded his beloved house, and later guarded her and protected her to the best of his ability and waited for her to join him after death.
Good drama, impossible, but good drama. The happily ever after ending there is terrific except for the fact that Mrs. Muir dies of old age.

The other type of ghost is sort of useless for a story, but great for tourism. This spirit runs over and over the event that led to its demise. Like a film or recording of the moment, it does the same thing each time. It is not distractable, it is not capable of interaction with living beings...it just keeps doing the same thing. There's a movie with Liam Neeson and Darryl Hannah as ghosts and Steve Guttenberg as the living person with Peter O'Toole as the proprietor of an Irish castle turned hotel that has ghosts in this situation. Until Steve G. disturbs their daily run-through, Darryl and Liam chase each other through the castle so he can kill his unfaithful wife every day at the same time. High Spirits? Something like that? They run through the exact same scene constantly...it never varies. To make the story different in the case of the movie, the living hero gets between the murdering husband and the victim wife and prevents the completion of the routine. This brings him into the routine and disturbs how it is supposed to go, thus allowing for interaction between the now three characters, the two ghosts and the living being.
It doesn't happen like this.
It's like a continuing tape that goes back and plays all over again. Energy is required, but not as much as to allow for interaction.
Where does the energy come from that allows the ghost to materialize?
Why, from the viewer, of course. Or from individuals in the area or household.

Something seen in a cemetery...that energy would definitely have to come from the viewer. Gettysburg ghosts need energy from people walking over the battle scene to appear. And they are fleeting at best. Never in the same place twice as long as the viewers aren't in the same place twice. If you're on a ghost tour and you are brought to the "scene" of the apparition, you'll be standing where the ghost can tap into your life force and no wonder the ghost appears.

There are revenants, too. These are wisps of ghostly beings or even animals, that draw on living energy to appear. See, ghosts can't appear if there's nobody there. They don't walk the halls when there's no energy to draw from. Can't do it. You set up cameras to capture spirit photos and nothing happens unless your own energy is still present somewhere, or the ghost can tap your energy and use it later.