Alcidamas De Soph (On the Sophists)

from http://www.classicpersuasion.org/pw/alcidamas/alcsoph1.htm

(18) To learn extemporaneous speeches is, in my opinion, difficult, and the memorizing likewise is laborious, and to forget the set speech in the trial of a case is disgraceful. Everyone would agree that it is harder to learn and commit to memory details than main heads, and similarly many points than few. In extemporaneous speech the mind must be concerned only with reference to the main topics, which are elaborated as the speaker proceeds. But, where the speech is previously written, there is need to learn and carefully to commit to memory, not merely the main topics, but words and syllables.