That is true. But also covered in my last point about people wearing masks to an extent. If you have a limited amount of time with someone you can be fooled but a mask "can" slip if you really get to know someone. But yeah it's not fool proof and people can definitely be fooled. But having an understanding of the power dynamic between yourself and the other person can help.

But yeah some people can hide negative emotions (e.g. sadness) very well.