True credibility is something you have to build. You have to develop a consistent track record of saying and doing smart things. It's a long, hard road to build that trust.
It's only natural that people look for short cuts.
Beginners and students, acutely aware of their own limitations of experience or knowledge, often to buy credibility. There's two common tricks that people try.
The first is to pull out the swanky clothes. This is not a terrible thing to do, since it usually helps make the speaker feel better and more confident. But if you are with audience members that know you, there might not be that big a bump in credibility. When I teach a seminar class, I can usually tell who is presenting that day, because they're dressed nothing like the way they've dressed in the previous month. Instead of track pants and hoodies, suddenly there are skirts and shirts that might comfortably hold a tie.
The second is to talk in a way that a person normally does not talk. This is nicely spelled out in this post about writing, but it's just as true for presentations (original emphasis):
(S)tudents tend to make the kind of mistakes in the formal research paper that they do not make in informal writing (such as blogs) that the sociolinguist William Labov found among working class speakers aspiring to be middle class: use of the word “whom” in situations where it is ungrammatical but sounds fancy, use of semantically incorrect but pretentious vocabulary (“Thesaurusitis”), longer sentences that lack punch but sound “upper class,” lack of demonstrative language, vague construction that lacks a point (“In this essay it shall be argued that...”).
A student giving a jargon filled talk is reacting much like a blue collar worker trying to fit in with a bunch of suits.
Of course, we know how this turns out. We've read this book and seen this movie: George Bernard Shaw's Pygmalion and its musical offspring, My Fair Lady. Henry Higgins tries to buy credibility for Eliza Doolittle by teaching her received English. But even as her pronunciation improves, she still gives herself away with every word she says.
You can fool some of the people some of the time when you try to buy credibility. But if the veneer scratches, even a little, things can fall apart almost immediately. This is particularly true in academic settings, where most of the audience has been highly trained to question, pry, be on the lookout for bullshit, find the weakness in the ideas and arguments, and attack any helpless underbelly.
When you try to buy credibility, the price you pay is in your authenticity. And I'd say that's also pretty important in giving a great presentation.
The Zen of Presentations, Part 37: What makes a good speaker?
Do you like me? Or any scientist?
The Zen of Presentations, Part 29: The shirt on your back