No I don't speak my mind, nothing like it. It's just way too risky, I get so many negative thoughts, it wouldn't be fair to share all that - most of it turns out to be rubbish anyway. Yet I can barely stop myself from unloading my entire knowledge on a subject when it comes up - I guess that kind of content isn't likely to be offensive exactly.
Ordinary people hide a lot of bad stuff too, a lot of them are extremely polite. Sales people probably feel like strangling awkward customers but they don't usualy let it show. People are more critical of each other when they're not around to hear it. Human ego is sensitive, perhaps too sensitive to deserve brutal honesty.
I don't know if I speak my mind less than the average person. Certainly I feel the strain of not doing. When I'm safely away from them I can articulate my criticisms of them in a way that seems quite civilised, but when I'm with them the same words seem a little too prickly.
Alcohol is reknowned for switching off the diplomatic faculty, if anybody wants to experiment. But I wouldn't recommend it.