The Look and Sound of Credibility
One source suggests that sixty percent of the impression you create has nothing to do with the message you send, but what you are nonverbally communicating - people pick up on nonverbal cues.
Police officers are exceptionally good at this: they develop a sense of when something is "not right" by the way a person acts or sounds. Interviewers hone their lie-detection skills by evaluating whether a person's tone, posture, expressions, and so on are incongruous with what they are actually saying.
But even layman have some degree of skill and perception. We form an opinion very quickly, as to whether a person "looks smart" or "sounds confident" - and it is from these conclusions that an overall impression of a person is formed.
The author avers that you can't not communicate: if you are visible or audible to others, you are transmitting cues that others will receive. If nothing else, they will notice your reluctance to communicate, and that itself tells them something. This is especially true when you are receiving (listening to) communications from others: you will react, consciously or unconsciously. Your only choice is whether to communicate in a practiced and controlled matter, or merely let it occur unconsciously.