Explaining Casualness: Truth vs. Common Knowledge

If an author or a person tells a story in which details are mentioned casually, and if some of these details appear to be true, does that confirm the truthfulness of the story? Should we instead think that the author was just counting on common knowledge (or common belief) for some of his details, therefore using them casually without explanation, while making up the rest of the story?   Here I discuss this type of attempted response to some undesigned coincidences and some external confirmations. "Oh, well," says the debunker, "The author was casual about that part because he could count on his readers and hearers to make the connection to this other thing just in virtue of common knowledge. It has nothing to do with artless truthfulness. He could have been deviously making some subtle connection to something he knew they'd get all on their own, even if he didn't mention it."  How can we tell when this is not a good response?  I use a dialogue I had in e-mail about the feeding of the five thousand as a springboard for discussing this topic. See others under the "casualness and independence" playlist. https://www.youtube.com/playlist?list=PLe1tMOs8ARn0NbWCs84HuF86v7ZnnUna5