The commandment tells us to speak truthfully whenever it is appropriate for us to speak at all. Respect for truthfulness does not compel us to reveal our minds to everyone or on every occasion. The Ninth Commandment assumes, no doubt, a situation that calls on us to speak. It does not ask us to tell the people at the next table in a restaurant that their manners are repulsive. It does not obligate a nurse to contradict a physician at a sick person's bedside. Nor does it require me to divulge all of my feelings to a stranger on the bus. We are called to speak the truth in any situation in which we have a responsibility to communicate.
Further, the command requires only a revelation that is pertinent to the situation. A politician ought to speak the truth about public matters as he sees them; he does not need to tell us how he feels about his wife. A doctor ought to tell me the truth, as he understands it, about my health; he does not need to tell me his views on universal health insurance. A minister ought to preach the truth, as he sees it, about the gospel; he does not need to tell the congregation what he feels about the choir director. The commandment does not call us to be garrulous blabbermouths. Truthfulness is demanded from us about the things that we ought to speak about at all.
Mere Morality, by Lewis Smedes