A ghost is a spirit or soul that remains after death. They can be benign or malevolent. They are often associated with a particular place or time and may have unfinished business that needs to be resolved.
People are naturally curious about the unknown and the possibility of an afterlife. Stories about ghosts have been so popular for centuries because they satisfy this curiosity while also often making a subtle commentary on life and the human condition.
It’s important to decide what kind of ghost you want in your story. Using the common mythological categories of banshees, poltergeists, mylingar, and so on is fine but try to be original with how you describe your ghost. A chatty spectre droning on about its death and woes might veer into comedy or family film territory, while an evil ghost is guaranteed to scare your audience.
The ghost’s motivation will be the narrative engine that drives your story. Usually the goal is to get revenge or bring justice to those who killed them, but ghosts can have any number of motives. They can manifest in a variety of ways and be seen or unseen, as a misty form or in a physical body. They can show themselves in a human-like way, make noises, touch people or even emit odours to let people know they are around.
Ghosts aren’t the easiest of subjects to work with because they are typically invisible. You must create a tension between your reader’s need to believe in the supernatural and the limitations of tangible proof.