That's probably not a fair question, given that you're an American. There's immediately some difference in expectations.
A better question is what happens if a young college grad in Japan chooses work no unnecessary overtime?
A better question is what happens if a young college grad in Japan chooses work no unnecessary overtime?
Just to give you a taste of this: In a previous job in Japan there was a work party on the weekend. I told my boss "I can't got, I have something to do" (A very normal way to refuse something in Japan. The expectation is that no one will ask what your vague reason is). This boss tries to pin me down on exactly why I won't be spending my day off on this event. This leads to 6 months of going over my work with a fine tooth comb, review periods and so on.
I'm surprised your boss tried to pressure you for info. Usually the companies that are more traditional ('non-mandatory' aka mandatory outside activities) will follow cultural norms and just do the punishment without the asking.
N=1 anecdote from a traditionally managed Japanese company.
If you (a twenty-something Japanese man) happen to say お先に失礼します。(announcing you're leaving) at 6:00 PM in front of the company's CEO during a site visit, and the CEO says, in a shocked voice, "I suppose you have somewhere more important to be right now.", and you say "Yes, I'm going on a date.", you can look forward to six months of nuclear-tipped ostracism from your colleagues and, when you complain to management about it, they will tell you "[Foreign employee] is more Japanese than you are; ask him."