Some of the most memorable parts of Martin Luther King Jr.’s speech – including the ‘I Have a Dream” segment – were not in the original draft. He ad-libbed them on stage. Gospel singer Mahalia Jackson, who was sitting behind him on the dais, admonished King to “tell them about the dream, Martin.” Mr. Branch has written that it isn’t known if King heard her admonition but that he later said he had forgotten the rest of the speech and took up the first string of oratory that came to him.