I read this about 40 years ago (another one, yes), but I believe the book was a few decades older than that. Probably Asimov's or Ray Bradbury's.

It's a short tale; some influential people need to lead the general crowd, but are unable to do so. They find a man with an outstanding authoritative figure, but he's a dumb man, who is emotionless, and can't speak properly to the crowd; a man with no true personality or soul. So they prepare scripted speeches for him, with hint marks regarding intonation and emotion, and teach him how to read in front of a large crowd.

The experiment goes very well; he manages to read perfectly and follow the scripts, and the crowds are highly motivated by him. But as he grows more accustomed to public speaking, he starts going off-script and speaking his own mind to the crowds. He then becomes a fierce dictator and acquires control of the entire country.

Does anyone remember which tale is this? Also, is this based on real world speech-reading techniques?