you mean a walk in? spirits inside yourself? the Warrens calculated the three steps of demonic possession. article here if you want to read about it https://getridofghosts.org/the-3-stages-of-possession/

i can't say for sure what is true or not. all my personal experiences has taught me is that I could tell there was a bad energy around (this when I was really open) and I could see how it influences people around me and they mistook it for their own energy, but I saw it coming from the outside and I saw it as a being (now good or bad I cant be the judge of that only that it gave off a bad feeling). I get if nobody believes this experience. I have difficulty believing it myself so it's fine. But there is something about me and energies, I can tell even if there are walls in between and so on (long story). it has happened that if someone has a very high emotional attachment to an object, even a piece of paper, I will detect where the missing object is, it is as if it has one energy and it would not go off like it does if it was not that the owner was calling it by it's own strong energy (the mother energy). Sorry if I sound crazy.

in short I think energies are around us all the time and just as we are showing up in their world they are showing up in ours, but it is only us who see that they go into our world, but I think especially the low energy is really close to us here on earth if we don't work on getting up our energies, we have the power to do that, our mind is more powerful than we think, I guess.

If you are worried that good or bad entity is inside your body I would say before that happens there are lots of warning signs and it too says you give your permission for it to do so. So influencing is one thing and possession is another. I have no experience with possession to my knowledge so I can't say anything about it, I'm just most familiar with energies.