The quiet one is often guilty.
Note: “Secret guilt by silence is betrayed.” — John Dryden, The Hind and the Panther . It takes great skill and strong nerve to tell a lie, even a lie of omission. Consequently, the guilty will often hold their tongue.
Sometimes the guilty conscience will hold one’s tongue. “One that will not plead that cause wherein his tongue must be confuted by his conscience.” — Thomas Fuller, The Holy State and the Profane State. The Good Advocate .
Sometimes the guilty hold their tongue as a matter of jurisprudence. In the United States, for example, the guilty have an absolute right to remain silent.