It's typical in my field that after we post to the arxiv, people from the community reach out to the authors to ask questions about the paper. Every time this has happened to me, I've responded with more explanation to try and address their specific questions. Sometimes it does seem like the person is arguing with the central results of the paper, and frequently the communication extends past what I'm willing to do (asking us to generate more data or wanting additional plots beyond what are in the paper) and I usually decline these requests. I don't really have the time, and I also don't like the idea of sharing plots/data over private communication. I would be happy to show additional plots in a more public forum, like for example in a seminar talk, where it's clear where the work originated, and where I get some credit for the labor I've done in preparing the presentation.
Recently I've been wondering: what if someone participates in such an exchange, and is then tapped to be a reviewer of our work during the journal submission process? Is this ethical? Should they disclose that they had informal previous communication with the authors? Should I factor this possibility in when I think about how accommodating I should be?