Every communication is two conversations, the verbal (the content) and the non-verbal (the body language).  When the two are aligned, a speaker can be persuasive, even charismatic.  When the two are not aligned, audiences believe the non-verbal communication every time. 

This insight has important implications for public speaking.  Chief among them is that you cannot give a successful speech with the best content in the world if your body language is disconnected from the content.  The audience will believe the body language. 

We are all unconscious experts in body language.  What we do is interpret it at an incredible rate of speech (actually faster than conscious thought) in terms of the intent of the other person.  Presumably this ability evolved from our caveperson days when our survival depended on it.  Nowadays, it doesn’t as often, but we still react instantly to perceived threats, sudden movements, and the arrivals of strangers. 

This means that, if you’re a speaker, you have in front of you an audience that is constantly construing your behavior in terms of its own needs, concerns, and fears.  So what matters most in body language is how you’re interpreted, not what you mean.  I’ve had clients that have said, “It’s just comfortable for me to stand this way in front of an audience,” and I’ve had to explain to them that comfortable for them may be insulting, off-putting, disengaging, or even simply defensive for the audience. 

Get yourself filmed while rehearsing an important speech and study your own body language.  Is it engaging?  Does it connect with the audience?  Do you look comfortable, glad to be there, and eager to talk to the group? 

It’s your job as a speaker to manage both conversations you’re having with the audience, the verbal and the non-verbal.  There’s lots more to be said about how to do that, but the basic point is that you are responsible for both your content and your body language in front of an audience.  You need to pay close attention to both in order to be successful.