That's very true too!

People can be married to someone and never know them well if they are not open to intimacy, secretive, private, reserved, or whatever. Some may share a lot of information but little emotion. Some may share a lot of emotion but little information. Some may share information but not opinions of you or politics or whatever. Some reveal more of their character than others. And still with some people, what you see is what you get! There are many variations.

I'm sure how well someone knows their T varies quite extensively depending on how your T works, her personality, other factors. I've had different Ts who I knew more or less about. This one I hardly know anything about.