Even if you manage to hide your dangerous thoughts perfectly, there is still the problem that duplicity is very unpleasant for most people, if anything because it requires constant caution and self-discipline to watch your mouth.

I agree and add that watching your mouth is not nearly enough. I, for example, are extremely good at watching my mouth when I am attempting to tow absurd party lines but I am irredeemably poor at controlling all the minute details of body language that must go with it. The only reliable way for most people to tell lies with adequate sincerity is to lie to themselves first.