Many factors influence the perceptions of another individual’s credibility. James P.T. Fatt examines nonverbal influences that impact three characteristics of credibility: competence, trustworthiness and dynamism. Perceptions of trustworthiness and competence are directly impacted by the amount of eye contact sustained as well as eye behavior. The correlation is so strong in fact, that Fatt identifies eye behavior as the single most influential factor of one’s credibility. While word choice is influential, dynamic uses of paralanguage, including vocal volume, rate of speech, pitch and pronunciation have a stronger impact on the perception of competence, trustworthiness and dynamism than the actual words. Strategic placement and use of surroundings will influence the credibility and perceived leadership qualities of individuals.

Fatt, James P.T. "It's not what you say it's how you say it." Communication World 16 (1999): 37-40.