I've spoken in front of audiences - large and small - more times than I can remember. Some of my talks tanked. Badly.
Most go really, really well. And the difference between the tankers and the good ones is one thing - a belief in what I'm saying.
It can (and often has been) an openly hostile audience (I've had people unexpectedly sit in just because I was "the guy from Microsoft", and that presented them with a rare opportunity to heckle). And most times I win those over as easily as the ones that are open to what I have to say to begin with.
And it's quite simply that when you believe in your message, when you just know you're right/your approach is right/your message has integrity, that you appear authentic.
And authenticity is very compelling, as a speaker.