Okay let me think: for me it is like trying to describe the color green to someone who cannot see -hard to put into words.
*intense thinking *
When I say chemicals, I mean literally the natural way the body works, nothing fancy. I am a medical professional, so if something I say doesn't make sense, let me know.
When the a person or other animal is frighten the body reacts. The sympathetic nervous system kicks in and prepares the body through cascades that release lots of chemicals (i.e. hormones and neurotransmitters etc) that tell the body's parts what to do: dilate the pupils, protract the eye ball from the sock, increase the heart and respiration rate, divert blood from the internal organs to the muscles, start preparation of energy for muscle use, etc -all these things prepare the body for either fight or flight. For a complimentary example: hormones are released during every second of the woman's life and which hormones dictate what part of her menstrual cycle she is in, and there are effects from that we can observe in the physical body such as menstrual bleeding. Same thing.
When the body is in a state of normalcy (emotional contentment) the body I would say is running at "cruise control" -the normal everyday quiescence needed for normal locomotion, respiration, normal hormone production, digestion, etc.
However, when the body is frightened, that sympathetic nervous system starts many different things in motion: I believe that as a person who is highly sensitive to other people's emotions, I can feel the very tiny vibrations (i.e. like a machine with lots of moving parts you could hear in a factory) made as the body "restructures" itself for the fight or flight response.
I cannot see auras, so I have never really thought about it; however it makes sense to me. The body is tuned to take in stimuli around it and process them. It makes sense to me that people who see auras are processing vibrational theory through sight instead of sound or touch.
When I refer to out of body, it is a totally different thing than personality. Out of body, in my words, means removing or expanding the energy (soul) out of the physical body. My faith believes that when someone dies, the physical body dies because it is aged organic material, but the energy (soul) remains. Some people think of this as spirits, guardians, ghosts, or angels depending of their faith.
For me the people easiest to feel don't necessarily have an intro or extroverted personality; it is more about a person's energy. So when I cannot feel someone as well, I believe they either 1) don't have major emotional shifts that set off the body's cascade of defenses, or 2) they project their energy out into the world less. It is kind of like people who talk very softly are easier to tune out than people who talk very loud -some energies (I call them a person's "energy signature") are easier to tune out because they aren't so obnoxious
This is simply how I experience the world. If you asked someone else "how do you experience the color green" they may reply differently. I hope it helps you work through some of the answers you were looking for all the same Have a lovely day!