There's the person reporting their experience, interested third parties judging that report, and then there's theory.
A person might very well distinguish, within their own thought, between a part that is reliable and a part that isn't: "I saw something, but it was hard to tell what at that distance, might've been a man." They might also distinguish between a part that requires justification and a part that doesn't: "He was standing right in front of me. I knew exactly who it was because I had just seen him at the diner and he had that same tie on." It might be a little harder to distinguish what is interpretive and what isn't, because language, but sometimes people will resort to a sort of homegrown phenomenalism: "I could see a patch of dark red on the carpet, it was shiny, and appeared to be spreading." That's interpretive insofar as it's English, but minimally so, I would say.
How an interested third party would determine whether someone's report is credible might be very different from the process of making the report with the intent to be truthful. An individual may have sole access to their own experience, but there are things others can see clearly about a person that they can't. If I'm a confabulist, it could be everyone knows it but me. I could be doing it right now!
I think for the purpose of developing a theory of "eyewitness accounts" or of self-reported experience we want to be able to take in both the perspective of the one making the report and those interested parties who might judge its credibility, and we might draw the boundaries differently from either of them. We might, for instance, claim that
everything about self-reported experience requires justification, but the part people often think doesn't is just the part for which they usually do have justification. ("He was standing right in front of me" could be taken either as needing no justification or as the sort of case where justification comes readily to hand.)
Back to our example. You said of the feeling that I am being stared at:
It doesn't strike me as being quite like anger, for example, which is more instantaneous and recognisable. The feeling you describe seems to be fundamentally something more basic, a funny feeling, which is then interpreted to be something more specific, more complex. — Sapientia
I'm going to say that "I felt like someone was staring at me" is as complete and honest a description of my experience as I can give. If you ask, "What does it feel like to have someone staring at you?" I will not know what to say; this is just what it feels like. I may know perfectly well that having this feeling is not evidence that someone is staring at me, but that doesn't matter. If I have a neurological condition and one of the symptoms is this feeling, which patients with this condition often report, then my neurologist doesn't care whether anyone was staring at me. He'll be satisfied by my report. If something I consider embarrassing just happened and I had this feeling, I would say that's why I had the feeling, but that doesn't affect my description of the feeling at all.
None of that is theory; that's just the data. How do you see the boundary here between what is interpretive and what is more basic, the inchoate funny feeling?