Your reading

The one sentence let out to someone who would never understand

Out of every character you could have named, you named No-Face. What gets read here is that choice itself.

What you were looking at

What you reacted to in No-Face was not fear, and it was not strangeness. If that were all it was, what stayed with you would be the shape itself. What you remember is probably not the shape. It is the one thing that came out of it, once.

No-Face never expected to be understood, not from the start. No-Face knows the outline does not fit anywhere and has given up trying to explain it. Someone who has given up usually stays quiet after that - permanently. But in that one scene, toward someone who was never going to get it anyway, the mouth opens. Once.

It might not even be a plea for help. Something that had been kept inside simply took shape and came out, a single time. What held you there was the exactly-once part of it. Not someone who keeps insisting - someone who lets it out a single time and then goes back to holding it. That is why the one time has not left you.

What you were watching was not the pain of being misunderstood. It was the content of the thing that came out anyway, despite that. Which means you are not drawn to what is strange or frightening. You are drawn to what someone says only after they have stopped expecting to be heard.

And where that leaves you now

One remembered picture gets added here. What you picked was the face that already knew it would not land, opening its mouth anyway. That is the position of the one speaking.

You are probably also someone who stops in the middle of explaining things. Not for lack of words - because you can already calculate the cost of finishing against how little of it will actually get through. So when you go quiet, it is not that there was nothing to say. It is the silence that comes after you already ran the whole exchange once, in your head, and decided against it.

One thing worth checking. Was there something recently you swallowed because you thought, they will not get it anyway? If that lands on one particular person, is it that you had already stopped expecting anything from them?