I don't believe in ghosts but as a kid I did a lot of reading about them and found them to be a fascinating part of many legends and folklore.

In many cases the idea is that ghosts are projections of spirits from another plane of existence and they are rather limited in how much they can interact with our physical world. They are doomed to be trapped in a mental and emotional prison of their own making. They will travel the same paths over and over again as they remember them. They will continue to function in the same manner even if the old place they are haunting is torn down and something else with a totally different blueprint is built in its place.

For example one might continue to haunt an old train station that's been replaced by a casino. They will use stairs and doors that no longer exist in our world. There's also the old "ghost train" that will continue to show up even though the tracks are long gone. Lots of examples where history is frozen for the ghost, and it's damned to repeat itself for eternity.

Otherwise they'd just fall through the floor or get sucked up in the HVAC. :)

It's not consistent, however. Other types of spirits will torment the living, and seem to be attracted to their spirits which they can see. They'll chase people, try to scare them, and so on.

Poltergeists will throw books and other objects around and in theory could harm or even kill you by pushing you off a ledge or similar. These types of ghosts were very evil people when alive. They are jealous of the living, and tend to have more "power" in our world because of their relentless wrathful nature.

In some stories the ghosts were witches, warlocks, or just dabblers in the occult. They unlocked some secrets about the afterlife, and are therefore able to have more agency in death than the average stuck-in-a-rut pedestrian ghosts. They usually have grander more malevolent schemes than spying on people taking showers. They may be limited on where and when they can appear as part of parameters of the ritual or "magic spell" they performed in order to come back to this world after dying.