Sounds like Ignition Point! by Isaac Asimov, originally published in The Winds of Change and Other Stories in 1983.
From Tropedia:
"Ignition Point!" is about a man who figures out how to write carefully constructed content-free speeches that will get audiences fired up. In the first test, the speechwriter stops in the middle, throws away the speech, and starts improvising — the speech worked on him, too.
The full story can be read via the Internet Archive preview of The Winds of Change and Other Stories.
It begins with a discussion between two men: Nicholas Jansen and Anthony Myers. Jansen explains that he's developed a computer program designed to weave certain key words and phrases into a written speech, which can have a rousing effect when that speech is delivered to an audience.
“What I have developed is a series of words, phrases, sentences that induce reactions in specific groups of people, divided by sex, age, ethnic groups, language, occupation, place of residence or almost anything else conceivable. If you could describe the audience your man would be addressing in sufficient detail, then I could supply you with precisely the sort of thing his talk should include. The more we know about the audience the more accurately my computer program can produce the key words and phrases. They are woven into a speech—”
Myers is doubtful, since he regards the man he wants to deliver a speech for him -- Barry Bloch, or "B.B." for short -- as impressive looking, with a good voice, but brainless. Nonetheless, Jansen insists that his program can enable even a "nincompoop" to ignite an audience.
Myers drummed his fingers on the desk softly. “Look. Let me explain about my man. He looks impressive. He’s got a good voice. He’s amiable and likable. Properly handled I can make him a corporation executive, or an ambassador, or the President of the United States. The trouble is he has no brains to speak of and he needs me to supply them. But the one thing he has to be able to do without me is to deliver a speech in such a way as to fool people into thinking he does have brains. This he cannot do, even if the speech is written out for him. The speech may be intelligent and yet he can’t say it in such a way as to make himself seem intelligent. Do you think you can write a speech better than I can?”
“Not better. Just foolproof. I can make it possible for him to push the right buttons and ignite the audience.”
Myers has Bloch practice reading aloud from carefully annotated manuscripts.
The manuscript presented to him on this day seemed worse than all the others. He looked at it in dismay. “What are all these marks?”
“Well now, B.B.,” Myers adopted the soothing tone he almost always used with Bloch, “just let Mr. Jansen explain.”
“It’s direction. It’s something you must learn, but it won’t be difficult. A dash means a pause, an underlining means an emphasis. A downward arrow before a word means you let your voice drop a couple of notes; an upward arrow means you let it rise. A curved arrow means you let things fade off in contempt, if it curves downward. If it curves upward, your voice rises in anger. A parenthesis means a small smile; a double parenthesis means a grin; a triple parenthesis means a chuckle. You never laugh out loud.”
Bloch struggles at first, but gradually improves with each additional day of practice. Jansen observes that he has a "certain potentiality".
They spent an hour over the first three paragraphs before calling a halt.
Myers said, “It's awful.“
Jansen said, “How did you do the first time you tried to ride a bicycle?“
Bloch repeated the speech all the way through twice that day; twice more the second day. A second speech was prepared, not quite the same, but just as empty of real content.
After a week, Bloch said, “I’m getting the hang of it. It seems to me that I’m getting so that it sounds good.”
Myers said, with hopeful hollowness, “I think so, too.“
Jansen said to Myers afterward, “He's doing better than I expected. He's got a certain potentiality, but--“
"But what?"
Jansen shrugged. "Nothing. We'll just have to see."
When Jansen feels that Bloch has made sufficient progress, he decides it's time test him out against a "homogenous" audience, for analysis purposes. Myers suggests the American Association of Textile Weavers. When the time comes to deliver the speech, Bloch steps confidently up to the lecturn, and gets off to a good start.
Bloch smiled at the audience and began slowly. (Don’t wait too long to speed up, B.B.)
He didn’t. He quickened the beat. At times, he stopped briefly to puzzle out .a symbol but fortunately, that sounded like deliberation, the kind of thought you would expect of mature wisdom. It helped to have that appearance.
Then he spoke still more quickly and emotionally and, to Myers’ surprise, he could feel the drumbeats start. There were those key phrases, with just the right kind of emphasis, and in response, he could feel the audience stir.
In fact, Bloch is feeling so in the zone, that he eventually decides to toss his script aside, and speak from the heart.
Bloch had paused briefly in his talk—just long enough to tighten the audience into a knot of tension—and then he brought his hand down savagely on the lectern, picked up the manuscript in a crumpled mess, and threw it aside. “I don’t need this,” he said, his voice rising into a distinct note of triumph. “I don’t want it. I wrote it in cold blood before I had you all before me. Let me speak now from my heart, as it comes to me, standing here before you; let me tell you all, friends and Americans, you and I, together, what I see in the world today and what I want to see, and believe me, my friends, the two are not—the—same.”
There was a roar in response.
Myers doesn't think Bloch can stay afloat without the script, but he's quickly proven wrong. Very wrong.
Myers clutched at Jansen wildly. “He can’t make it on his own!”
But he could and did. He spoke through and over the applause and the shouting. It scarcely mattered if he were heard. He raised both arms as though to embrace the audience and a voice shouted, “Go on! Give it to them!”
Bloch gave it to them. Exactly what he said scarcely mattered but when it was over, there was a wild and jubilant standing ovation.
After the speech is concluded to a standing ovation, Myers is confused. Jansen explains that when an audience is ignited, they in turn can ignite the speaker, and in such a state, a speaker holds enormous sway.
“What happened?” said Myers, through the noise. (He was applauding as loudly as everyone else.)
Jansen remained seated, in a strange attitude of collapse. He clutched at Myers, drew him close, and said in a shaking voice, “Don’t you see what happened? It was a one-in-a-million shot. Just toward the end I began to wonder if it were possible. It can happen—”
“What are you talking about?“
“The audience ignited and Bloch was speaking to an ignited audience for the first time in his life, and speakers have their ignition point, too. Bloch himself ignited, and an ignited speaker can carry public opinion and move mountains.”
“Who? B.B.?“
“Yes.“
To Myers, this sounds like a good thing, a success, but Jansen fears they may have created a monster.
“Well, that's great.“
“Is it? When ignited, he’s got power, and if he finds out he has, why would he need you? Or me? And if so, where will he go? There have been great charismatics before who have not always led to glory.”
Bloch was with them, people crowding about him. He said to Myers in a breathless undertone, “That was easy! I feel great!” He turned to those about him, laughing, holding them all with no trouble.
Myers looked after him, confused; Jansen looked after him, afraid!
