Posture is critical to public speaking.
The stance of a speaker says much about how a speaker feels about what is saying as well as how he wants the audience to receive the message. A speaker hunched over at the podium, or one who is furtively glancing sideways or upwards, but never at the audience, radiates discomfiture.
James Lowther, Speaker of the British House Commons early in the 20th century, gave this advice, “There are three golden rules for Parliamentary speakers: Stand up. Speak up. Shut up.”
The first two relate directly to posture, the third bit relates to common sense. And that is something no speaker can project enough.