Dialog like this is a challenge. Matheson’s main character, David Millman, is ultimately having a conversation with himself. He’s the crazy guy who hears voices in his head. I’ve written a couple of pieces that have this happen in them, but I haven’t been able to pull them off so well (in my humble opinion). Why is it that this story works but my own efforts haven’t?
I think it’s because when I’ve done it, my crazy main character sounds… crazy. I think one reason this piece is pulled off so well is that, through Millman’s internal dialog, the reader is presented with several plausible alternatives to him actually being crazy. The internal dialog is presented as a having a series of explanations that all have the appearance of an external dialog - some through technology, one as communication with the dead. All are direct dialogs - person to person. So this internal dialog never gets old, as a fresh perspective is presented with each new plausible explanation.
The other thing I found valuable from this story is how the title positions it. “Person to Person” is a model for dialog - Millman’s have a direct conversation with another person. But, the ending brings out the double meaning. Millman suffers a psychotic break (or some such - I’m no psychologist), and what’s really happening is he is transition from being one person to another.