Quintilian, and J.S. Watson. Institutes of Oratory. Edited by Lee Honeycutt, 2010. Kindle Electronic Edition. Book 10 Chapter 6.
Quintilian takes some time to emphasize the importance of careful consideration. It is possible to consider a case and the available arguments even when not writing (Quintilian X.6.1). Practice in thinking and mental review is necessary. Quintilian urges systematic practice to this end (Quintilian X.6.3). The practice of memorization does not need to govern the entirety of a speech. It allows for insertion of ideas or phrases which occur to the speaker in the moment (Quintilian X.6.5).