Why is it important for a speaker to read the audience's body language?

Public speaking is essentially one-way communication from the speaker to the audience. Mostly, but not all. The audience does respond to what is said and a good speaker must adapt to the audience's attitudes in order to improve his style and content. Thus there is a great advantage for the speaker in knowing what the audience thinks and feels because that way he can adapt his content and style to the audience, an adaptation that will improve the audience's interest, engagement and persuasion.

Unfortunately, it is not easy to know what the audience is thinking - apart from body language, the main ways are by receiving questions from the audience and directing questions to the audience. As we will see, these two methods are indeed very effective in certain areas, but limited and even dangerous in other areas.

Alternative A - to receive questions from the audience

Questions and interjections are useful in several aspects - they reveal for the speaker the weak points in his speech, they can focus him on things that the audience would like to hear more of, and they also provide an excuse for the speaker and an opportunity for engagement for the audience. On the other hand, the questions may represent only the minority involved (20% of the participants ask 80% of the questions) and therefore their emphases may be biased and not representative. Also, they only come occasionally so the feedback they give is neither continuous nor permanent. Finally, they take up valuable time.

Alternative B - to direct questions to the audience

It is an even more active approach of working with the audience, and it is more extreme in terms of advantages and disadvantages. Like receiving questions, it can also reveal weak points for the speaker, but it will not focus him on the problematic parts because the speaker obviously does not know what those problematic parts are (otherwise he would not need the questions). Asking questions is more representative because the speaker chooses who to address, but still most of the volunteers to answer are part of the active minority which is not necessarily representative. And they have certain harms of their own, and proactively asking the audience if the topic is interesting is a dangerous exercise (try it with high school students and see).

Alternative C - learn to read body language

This leaves us with body language as a means of sensing the audience. The great advantage of reading body language is continuity - we can know at any given moment what the audience is thinking without having to wait for questions. Another feature is universality - all the participants in the audience transmit their body language and thus we have the ability to read them all at the same time. A third advantage is that reading body language does not harm the flow of the speech and does not take up valuable time. Finally, reading body language is done even without the audience being aware of it, and this allows the speaker to pick up cues from the audience without telling the audience that he is doing so. Although reading body language requires learning and skill, with a little practice it is certainly possible to turn it into a natural skill whose value is priceless.