Quintilian, and J.S. Watson. Institutes of Oratory. Edited by Lee Honeycutt, 2010. Kindle Electronic Edition. Book 10 Chapter 7.
In book 10 chapter 7 Quintilian praises the ability to speak well without specific preparation. “But the richest fruit of all our study, and the most ample recompense for the extent of our labor, is the faculty of speaking extempore (Quintilian X.7.1). It is the honor of a man to be able to serve in emergencies. The orator needs to be able to respond to a situation just as a sailor deals with changing conditions (Quintilian X.7.3).
To gain this ability the orator needs to be able to organize ideas in a logical order at will (Quintilian X.7.5). The power of fluency is increased by practice in careful writing (Quintilian X.7.7). This involves mental dexterity as the orator is speaking while planning ahead (Quintilian X.7.11).
Quintilian cautions against the assumption that extremporaneous speaking is easy. It takes a great deal of mental effort and practice (Quintilian X.7.18). He also reminds the reader that some time for preparation is almost always availabke, even if it is a short time (Quintilian X.7.20).
Once the ability to speak extempore has been gained, it requires ongoing cultivation (Quintilian X.7.24). It can be practiced orally or mentally and deserves some attention regularly. Having a habit of care in words is also helpful (Quintilian X.7.28). In reality, most speakers engage in a combination of prepared and spontaneous elements (Quintilian X.7.30).