This study explores how truth-default theory (TDT) and relevance theory apply to public speaking and business communication. Two hundred eight college students watched one of four speech introductions varying by topic relevance and the presence of an explicit credibility statement. Participants rated speaker trustworthiness and credibility. Results showed that topic relevance had a greater influence on audience perceptions than explicit credibility statements. Highly relevant topics reduced suspicion and supported TDT’s claim that deception detection requires a trigger. These findings illustrate how speakers can build credibility and how audiences evaluate messages in educational and business public speaking contexts.