I don't really know about American manners, but Japanese people are generally very nice, and they're aware of others, and usually adapt themselves to the situation, and if they do disrupt you in any way they make a nice bow and everything is alright again:D). One thing I do know about Americans (a bit stereotypical but I seriously could see/hear who was American when I was in Japan), don't be loud. Wheter you are in a public park, on a train or metro, or in a museum, Japanese people tend to be very quiet, if not completely silent. Speaking about trains, I don't know if you've seen it or not, but on Japanese train platforms, there are special markers for people to line up in front of the platform gate doors. That's all I can recall about manners, I hope this is enough, and if not, other people will probably help:).
This is a really broad question that invites a lot of stereotypes as well I think. Do you have a specific situation where someone used different manners than you expected that you need more clarification for?
Just as an example, in Japan people generally take off their shoes indoors. I heard it‘s not done the same in all of America, some with shoes on inside and some not. How could you compare these shoe manners across America with Japan, of there isn‘t a single way of doing it in America?
Another example, in Tokyo people generally walk on the left side and in Osaka on the right side. It‘s polite to leave space for opposite traffic, but which is the polite correct side isn‘t the same everywhere.
I‘m not American, but I‘ve seen enough discussions about the shoe thing and any stereotyped comparison between America and Europe that can‘t be answered so easily.
I know that Japanese people say 頂きます(いただきます) before starting to eat and ご馳走様でした(ごちそうさまでした) after eating/ completing a meal.
There is also many unspoken norms that only exist in Japan with many such as giving business cards with 2 hands pinching it with the thumb and index finger, tips being an offence, arrive earlier and dont just make it with a minute to spere and many unspoken customs you might need to search up or directly ask someone who lives/ lived in Japan long term that would otherwise be unnoticed. There is just too many for me to list in one post
