Genesis 6: 1-4 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, 2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. 3 And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. 4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
This passage, in my opinion, describes the potential for otherwise spiritual entities to assume flesh. If not flesh, how else could they mate with humans?
I believe that it is plausible that fallen angels manifest on Earth, take the blood of a mutilated animal (cows, for instance), and use that blood to make flesh and walk among us. This may explain how they can physically interact with humans and disappear without a trace.
I also believe that when humans die, their spirits leave the Earth and enter into either Heaven or Hell. How, then, could our loved ones remain on Earth to communicate with those left behind? I believe that fallen angels can assume the spiritual form of the dead (after all, without flesh, would our spirit still retain the same human characteristics?).
Perhaps the entities knocking on walls or appearing in photographs are demonic in nature and are simply manifesting in order to mess with the minds of those seeking information about the afterlife (and give them a false hope of something other than what the Bible says).
I have also heard of theories about certain minerals or rocks (like sandstone) that act as a tape recorder. This may explain the residual hauntings where a “ghost” is trapped in a routine regardless of whether someone is watching or not. For those entities that interact with humans, I believe they are not, nor ever have been human themselves.
The account of Saul’s interaction with the “witch of Endor” is found in 1 Samuel 28. To paraphrase, Saul had banned the use of “familiar spirits” and “wizards” from Israel. However, when the enemy came upon him and God refused to answer his prayers, he went against his own decree and found a woman who had “a familiar spirit” who was able to conjure up the spirit of the prophet Samuel.
So, this presents Biblical evidence of the existence of not only what we would now call a medium, but of spirits that could manifest themselves in more than one form (the woman first mentioned that she “saw gods ascending out of the earth” to which Saul responded “what for is he of?” suggesting that there could be more than one spirit AND form).
Another interesting passage is found in Ezekiel 1. It is a vision, but when you read the description of the “chariot”, it sounds very much like it could have been a type of UFO. A vehicle made of large wheels the color of “beryl”-gold, aquamarine, emerald-descended from an amber colored cloud and was used to capture living creatures to be taken away by “spirits” with wings.
Finally, as to my own personal theory of the possibility of cattle mutilation leading to physical manifestations of these spiritual beings, Leviticus 17 is a chapter giving rules to the Israelites, and there are a number of verses in this passage that absolutely forbid the consumption of blood, because “the life of all flesh is in the blood.” I have taken that to mean that if a being has no blood, it is not alive (in the physical sense). Conversely, if a being obtains blood, it can be made alive. If a spiritual entity were to drain a living creature of its blood (surgically removing and spilling no drop, in most cases), is it completely impossible that it could consume the blood and become flesh? At least temporarily? Given the nature of the beings in question, I cannot rule it out, but I certainly have no proof other than probably grasping at straws.