Alcidamas De Soph 33 (On the Sophists)
From http://www.classicpersuasion.org/pw/alcidamas/alcsoph1.htm
(3) In the first place, one may condemn the written word because it may be readily assailed, and because it may be easily and readily practiced by any one of ordinary ability. To speak extemporaneously, and appropriately to the occasion, to be quick with arguments, and not to be at a loss for a word, to meet the situation successfully, and to fulfil the eager anticipation of the audience and to say what is fitting to be said, such ability is rare, and is the result of no ordinary training.
(4) On the contrary, to write after long premeditation, and to revise at leisure, comparing the writings of previous Sophists, and from many sources to assemble thoughts on the same subject, and to imitate felicities cleverly spoken, to revise privately some matters on the advice of laymen and to alter and expunge other parts as a result of repeated and careful excogitation, verily, this is an easy matter even for the untutored.
(9) I think, too, that in human life the ability to speak is always a more useful accomplishment, but the writing of speeches is seldom of opportune value. Every one knows that the ability to speak on the spur of the moment is necessary in harangues, in the courtroom, and in private conversation. It often happens that unexpected crises occur when those who can say nothing seem contemptible, while the speakers are seen to be honored by the listeners as possessors of god-like minds.
(11) Would it not be ludicrous if, when the herald announces, `Who of the citizens wishes to speak?', or, when the water-clock in the courtroom is already flowing, the orator should proceed to his writing-tablets to compose and memorize his speech? Verily, if we were tyrants of cities, we should have the power to convene the courts and give counsel relative to public affairs so as to call the citizens to the hearing after we have had time to write our speeches. But, since others have this power, is it not silly for us to practise aught save extemporaneous speech?
(12) The truth is that speeches which have been laboriously worked out with elaborate diction (compositions more akin to poetry than prose) are deficient in spontaneity and truth, and, since they give the impression of a mechanical artificiality and labored insincerity, they inspire an audience with distrust and ill-will.
(31) Furthermore, I am now essaying the written word because of the display orations which are delivered to the crowd. My customary listeners I bid test me by that usual standard whenever I am able to speak opportunely and felicitously on any subject proposed. To those, however, who only now at last have come to hear me (never once having heard me previously) I am attempting to give an example of my written discourse. The latter are accustomed to hear the set speeches of the rhetors and, if I spoke extemporaneously, they might fail to estimate my ability at its real worth.
(32) Apart from these considerations, it is possible, from written discourses, to see the clearest evidence of the progress which it is fitting that there should be in thinking; for it is not easily discernible whether my extemporaneous speeches are now superior to those I formerly delivered, as it is difficult to remember speeches spoken in times gone by. Looking into the written word, however, just as in a mirror, one can easily behold the advance of intelligence. Finally, since I am desirous of leaving behind a memorial of myself, and am humoring my ambition, I am committing this speech to writing.
(33) It must distinctly be understood that I am not encouraging careless speaking when I say that I esteem the ability to speak extemporaneously more highly than the written word. My contention is that the orator must prepare himself in advance in ideas and their arrangement, but that the verbal elaboration should be extemporaneous; this extemporaneous verbal exposition, in its timeliness, is of greater value to the orator than the exact technical finish of the written discourse.
(34) In conclusion, then, whoever wishes to become a masterly speaker rather than a mediocer writer, whoever is desirous of being a master of occasions rather than of accurate diction, whoever is zealous to gain the goodwill of his auditor as an ally rather than his ill-will as an enemy, nay, more, whoever desires his mind to be untrammeled, his memory ready, and his lapses of memory unobserved, whoever has his heart set upon the acquisition of a power of speaking which will be of adequate service in the needs of daily life, this man, I say, with good reason, would make the practice, at every time and on every occasion, of extemporaneous speaking his constant concern. On the other hand, should he study written composition for amusement and as a pastime, he would be deemed by the wise to be the possessor of wisdom.