Some people wear masks. Some people seem to speak, not spontaneously, but on a sort of internal tape delay. Their faces tend to fix in one expression: as often as not, a permanent smile; sometimes a neutral expression. There often seems to be a strain around the jaw. They are consciously controlling their faces, and it takes some effort.
If they laugh, you can hear that it is feigned, something they are imitating, not something from within themselves.
If they are not necessarily bad people, they are obviously people who feel they have something to hide. They have some sense of guilt; this may be deserved or undeserved.
Technically, these are “hypocrites.” The word, New Testament Greek, literally means an actor, or one who is wearing a stage mask, as Greek actors did.
M. Scott Peck calls them “People of the Lie.”
The one certain thing is that you cannot trust them.