In Stranger Things, early in the series,
Joyce finds a way to communicate with her missing son Will, using electric lights and an alphabet pointed on the wall (where Will spells out his answers by activating the lights).
However, later when we actually see more of the Upside-Down...
The environment seems to be relatively static, and as far as I could tell, completely empty of people. And, as far as I could tell (from the scenes that cut between the two perspectives in real time), the Christmas lights were not visible from the Upside Down. I didn't notice the alphabet painted on the wall either. From this one could assume that only things that have been in the same place for a long duration 'translate'.
But if this is the case, then
how would Will know a) that his mother was asking the questions that he subsequently answered with the lights, and b) that the lights/alphabet were there to be interacted with?
Is there any set of rules for how much somebody in the Upside-Down can sense subtler changes in the regular world, either extrapolated from within the show or given by those involved with production, that explains how any of that conversation could have happened, from Will's point of view?