One of the more challenging forms of public speaking is substituting for someone else at a technical conference. I have done it well just once in my career, back in the early 1980’s. There was a technical conference in Toronto. I was chairing a session on hydrogen embrittlement of steel. A French scientist had volunteered to give a presentation. However, he warned me that there was a possibility that he might not be allowed to travel.
A week before the conference an airmail envelope arrived with his text and slides. There was a brief note saying he could not come, and I should just go give his presentation. It turned out to be straightforward, because he and I had worked for the same PhD thesis adviser in graduate school. For about three years our desks were in the same lab and faced each other. We had built equipment together, and discussed that topic over and over. Later he had sent me many of his publications. A week was ample time to understand his presentation, because I already understood his approach to the topic.
A half decade earlier our thesis advisor gave a much more difficult substitute performance on the same topic. He was co-chairman of an entire conference. One of the keynote speakers could not make it because he had to testify in court as an expert witness. The manuscript and slides arrived about 2 hours before the presentation. Our advisor warned the audience that he was filling in at the last moment. Then he did an excellent job of communicating someone else’s material. How could he do that so comfortably? He and his co-chair had written a series of review articles on that topic. They already had read and discussed hundreds of technical papers. So, they understood what everyone thought, and could step into anyone’s shoes.