Even A Fool Looks Smart When They Don’t Talk - Proverbs 17-18

Proverbs 17:28 (NASBStr)

Even a fool, when he keeps silent, is considered wise;

When he closes his lips, he is considered prudent.

Proverbs 17:28 (ESV)

Even a fool who keeps silent is considered wise;

when he closes his lips, he is deemed intelligent.

Solomon might have known some extroverted fools.   He writes often about the tongue and how the foolishness of the fool is shown by how he/she uses their tongue. In the above proverb we see that a fool, who has no heart for God and even denies God's existence (Psalms 53:1), is still counted as wise if he simply quits talking.   He is prudent, or deemed intelligent, when he doesn't allow the folly in his heart to be seen by others via his tongue. The main canvas of the heart is painted by the mouth and the tongue.   Wisdom uses the mouth and the tongue prudently.  Therefore, if the fool does so as well, he will look wise.  Job says this about his three counselors, who are foolish in their counsel:

Job 13:4-5 (ESV)

As for you, you whitewash with lies;

worthless physicians are you all.

Oh that you would keep silent,

and it would be your wisdom!

He, of course, is not.  The fool can't change his stripes. A fool is a fool because he doesn't fear the Lord (Proverbs 9:10). Yet, he can fool us (no pun intended) as though he were wise by simply keeping his mouth shut.  Note that the last line of the proverb states, "he is considered prudent/inteligent."  He is NOT prudent, he only looks so because he doesn't let his folly out in the open.   This is the proverbial wolf in sheep clothing.  Peter warned us about them in the last days as they try to disguise and  dismantle the gospel message: 

2 Peter 2:1-3 (ESV)

But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction. And many will follow their sensuality, and because of them the way of truth will be blasphemed. And in their greed they will exploit you with false words. Their condemnation from long ago is not idle, and their destruction is not asleep.

The fool will always be the fool.   Don't be deceived when they are quiet.  Their folly will eventually come out.  See also:

Proverbs 17:7

Excellent speech is not fitting for a fool,

Much less are lying lips to a prince.