Anyway, at some point I happened to run into him briefly outside of the supermarket, at a party that happened to be Lady Gaga-themed, so the next time I was in his checkout line I brought that up, and we started talking about people going out in costumes. He mentioned something about how he used to dress up like Jack Sparrow and go out to bars; I said yes, I remembered him telling me about that bin a previous conversation. He then responded, "Well, then, I'm guessing I also told you about my dead fiancee?"
Um...no, you did not.
When I responded (very much startled) with something along that line, he proceeded to tell me a long drawn-out story about how his fiancee died suddenly and how he didn't leave his room for months and people thought he was in a coma, and when he finally did manage to get out of the house again, the only way he could interact with people was by wearing a costume and putting on a persona. All of this while there were other people behind me in line, waiting for their turn; I felt sorry for him if the story was true, but at the time I was mostly embarrassed for holding up the line and just generally at a loss as to how to respond to an information dump of that caliber.
I avoided his line for a while afterward, but there soon came a point where his line was by far the shortest and therefore the most logical choice. He didn't mention our most recent conversation and we went back to basic small talk, so I thought maybe that was a one-time thing and I just caught him on a bad day or something.
A few weeks later, the same thing happened. He started to tell me something he'd told me before, I mentioned that I knew that already, and he replied, "Oh, sorry. After the head injury and the stroke, my memory's not so good." This time the story was about how he was in some sort of car accident, had a major head injury but didn't get it checked out because he couldn't afford an ambulance ride, somehow survived a brain hemorrhage, and since then has been having seizures/blackouts at random times, but still hasn't sought medical attention. All of this delivered with an air of complete nonchalance while other people were waiting in line.
I didn't go to his checkout station any more after that, and soon thereafter I moved and didn't have to deal with that situation anymore; it made me incredibly uncomfortable trying to think up an appropriate on-the-spot response to these bomb-droppings. I'm not sure if the guy was a compulsive liar or if all this really did happen and he just had no filter/a complete lack of understanding of social relations, but whatever it was, it was weird.
(I thought it was just me he was doing this to at first, maybe as some sort of tragically misguided come-on, but after talking to some of my friends in the area it seems he does this with anyone who has more than a passing interaction with him. He was a nice enough guy outside of the oversharing problem. I don't know why I didn't think to mention this to a manager or something at the time - I think I was just too taken aback by the whole situation - but now I'm gone and it's a moot point.)
TL;DR Cashier thinks because he and I sometimes have conversations, it means he can drop tragic and dramatic, highly personal stories on me at any moment.