Can Creator give us a quick summation of the primary differences between a medium and someone who fantasizes readily, or is even delusional?

Here you are referring to common criticisms and skeptical perspectives of those who scoff at that notion that psychic mediumship is a real and valid pursuit. When people do not believe in the afterlife or a higher power, this will greatly restrict their acceptance of anything relating to the paranormal when it comes to things beyond the Earth itself and the state of being of living humans, but that is a very restricted view representing a quite severe state of ignorance. The inability of people to envision such possibilities, as someone talking to the dead, is the result of a closed mind and a limited ability to think freely. This is because such a state is engineered within most humans by interlopers wanting to control you and limit your perspectives to keep you subjugated. They want you to fear death and believe that you are limited, have a limited reach, a limited lifespan, and, "because you come from nothing," have no inherent value other than to your own ego. In part, this is because the interlopers do not support the divine realm and do not, themselves, believe in a higher power. So a skeptic, who cannot bring themselves to even consider the possibility a psychic medium might be speaking to a spirit in the light, must find some trivial way to explain the phenomenon on display. Many people simply assume it is all a hoax with mediums spouting platitudes and simple-minded commonalities that are nothing but an educated guess, but might persuade someone's relatives that they were, indeed, hearing from their loved one because most times the medium will know something about who they are trying to connect to, at least their gender, and maybe something about the passing even, depending on what was shared by the family in wanting to have a session done in an attempt to reach their loved one. Even with blind readings, these can be faked in some instances by a clever imposter, and this has been shown to happen in the past historically. If there is mention of something highly unusual and specific to a loved one who is departed, the skeptic may assume this was a hoax, that the family member was complicit and in on the scam, and perhaps is being paid to be an actor, to pump up the reputation of the would-be psychic. Some people are highly imaginative and may get carried away with themselves, and so some people see a psychic medium as someone who thinks they are possessed of an intuitive reach just through the power of suggestion because they themselves are gullible and, in believing this to be so, will walk the walk and talk the talk, and may have enough educated guesses to keep them in the game, at least for a while. The harshest criticism comes from those who believe that a psychic medium, who seems quite genuinely sincere in what they do, must be unbalanced and even delusional because if they are not perpetrating a hoax, some part of their mind is doing so to bring up visions or impressions to concoct a story that is not true. So it is likely, in the way of thinking of the skeptic, to be a mental aberration and might even be a diagnosable mental illness of some kind, if of a benign nature. All of these explanations are more likely than not to be false. There are some scammers, but most psychic mediums of any prominence are legitimate and doing the best they can with a fragile and uncertain intuitive ability to connect with people's loved ones and convey a sense of a message. It is the skeptics who are, in a sense, not in balance in being unable to entertain the possibility there is more in the world than dreamt of in their narrow-minded thinking, and it is they who are corrupted and limited, being under the control of beings, both dark spirits and extraterrestrials, who are telepathic and can see the akashic records of the past lives a person has lived, and use that as a way to stir up trouble with a deep part of their mind to worry about prior bad times returning, and so on, and carry out a reign of terror. So it is the doubtful and the skeptic among society who are perpetrating a scam, without realizing it, in representing non-existence of the paranormal as reality when in fact the opposite is the case.