Going back to Aristotle, rhetoric is the practical faculty of observing and deploying, in any given case, all the available means of persuasion.
What then, makes a message persuasive? A lot lies in the logical argument itself, the demonstration. But, quite importantly, Aristotle himself notes that knowledge does not automatically produce conviction!
In his words, “arguments based on knowledge imply instruction, and some men cannot be instructed.” Which is why he realistically lists not one but three modes of persuasion: the ethos (the personal character of the speaker), the pathos (the appeal to emotions, putting the audience in the right state of mind, creating a sense of togetherness, showing interest in them, starting with neutral topics all agree on), and the logos (the demonstration, plea, or logical proof itself).
To keep it short: in order to make an argument truly stick, you also need a few style “tricks”. Some of the required qualities of style for a persuasive message according to Aristotle are:
- correctness and clarity
- a free run of sentence.
If you want to really captivate your audience, deliver a message that is simple, unexpected, concrete (tangible), credible, emotional (contagious) and memorable (weave in fascinating stories).
In other words, make it atypical and unfamiliar enough to attract, but easy enough to understand. (Aristotle used to thing that metaphors are the greatest thing by far to master, since a good metaphor “implies an intuitive perception of the similarity in dissimilars”!)
To truly influence an audience, you need information that is vivid and readily available, and framed in a way that would spur people into action (fascinating research on framing by Kahneman et al. – see here – basically, framing the issue as a gain will lead to risk-averse behavior, while framing it as a loss will lead to more risk-seeking behavior of decision makers). Do not ignore social proof (for more about this, see Influencing People, courtesy of the University of Michigan), authority, scarcity and other pillars of persuasion.
And remember: how and when you deliver the message truly counts. The audience is more likely to consider a speaker credible and competent if they speak at a pace of around 190 words per minute (relatively fast), in a deeper rather than high-pitched voice, with suitable intonation, in a high-power body pose (open body posture), making eye-contact, using the space (get up and close, but not too close) and employing illustrative and positive hand gestures. And remember, people are way more likely to respond positively to you after lunch than on an empty stomach!