It is unlikely you will ever know what a Japanese person is REALLY thinking.
Smiling and being nice and friendly are most important and if they are unhappy with you it is unlikely they will ever tell you.

First you know of it will probably be when when they stop spending time with you.

If it is a Japanese partner it will probably be a very short email telling you they cant see you any more.

If it is a Japanese employer it might be a letter telling you that you are fired.

If it is a Japanese "friend" they will probably just stop inviting you places or making excuses not to accept your invitations.

Most likely you will have absolutely NO IDEA that any of these people were not 100% happy.
Even if you asked them the day before if they were happy with your relationship/work performance/friendship it does not mean that you will not get a short email the next day telling you it is over - with ZERO explanation.

It confuses westerners because we hear Japanese people go on about the importance of having a strong "heart" and being brave.. but when it comes to expressing any sort of discontent or telling someone how they REALLY feel, VERY few of them can do it.
I only know two Japanese who can.
One of them now lives in a western country because, in her words,
"I never know what the f*ck they are thinking!"
(This was a Japanese person talking about Japanese people!!!!)

And don't let anyone's good English fool you.
Maybe they have lived in other countries and speak excellent English and appear to understand western culture really well.. it means nothing.
When it comes to saying what they REALLY think they don't have the heart to do it!!

And, as you can tell by my post, it frustrates the hell out of westerners and hurts EVERY time it happens!!

"What the hell did I do?????????????!!!!!"