Here are some more extracts from Rhetoric at Rome, from Chapter V; Cicero’s Rhetorical Theory.
[In De Inventione] Cicero makes one of his characters, the lawyer Scaevola, dispute the claim of oratory to have civilised mankind. Was it not rather men of practical good sense, without any special gifts for oratory, who performed this function? Eloquence had in fact been actually harmful at times, as in the case of the Gracchi. Moreover it might be said that oratory was merely an instrument to serve certain purposes, ‘to make the case you are pleading in the law courts appear to be the better and more plausible, and to make your speeches to the people and the senate as effective as possible, in fact to make the wise think your speech eloquent and fools even think it true.’… In his practice he might use the arts and crafts of rhetoric to make the worse cause appear the better, and might boast of having thrown dust in the eyes of the jury, but in his theory oratory was purely a power for good. (p. 54)
Cicero is indeed less interested in the appeal to the head than that to the heart. ‘Men’s judgements’, he tell us, ‘are more often formed under the influence of hatred, love, desire, anger, grief, joy, hope, fear, misconception or some other emotion, than by truth or ordinance, the principles of justice, the procedure of the courts or the laws.’ (p. 58)
His main emphasis now is on the necessity for the orator to feel the emotions he tries to arouse. ‘It is impossible’, he says, for the hearer to feel grief, hatred, prejudice, apprehension, to be reduced to tears and pity, unless all the emotions which the orator wishes to arouse in the juror are seen to be deeply impressed on the orator himself.’ If anyone wondered how the orator could be constantly moved to anger, grief, or other emotions in matters which did not concern him personally, the answer was that the sentiments and topics he made use of had such power to move that there was no need for simulation. The very nature of the speech whose object was to move the audience would be such as to move the speaker more than anyone else. Like the actor, the orator would live his part. Antonius, who in this part of De Oratore serves as Cicero’s mouthpiece, records his defence of M’. Aquilius, and claims that his pathetic peroration came from the heart, and that when he displayed his client’s wounds the action was not premeditated, but inspired by violent grief. Speaking in his own person in the Orator, Cicero says much the same; in all his pathetic passages it was not so much his talent as his capacity for experiencing the feelings he expressed that accounted for his success. (p. 59)
From the appeal to the heart we turn to the appeal to the ear. The consideration of style occupies most of the third book of De Oratore, and though in the main the matter is traditional, it is worth noting where Cicero lays the emphasis. Of the four virtutes dicendi, the first two, ornate and apte congruenterque, are the important qualities. It is these that make men thrill with terror, gaze open-mouthed at the speaker, cry aloud and think him a god among men. Above all it is the ability to use ornatus that constitutes the crowing glory of eloquence. How is this adornment to be come by? The answer is that it will come of its own accord to the learned orator. ‘Rerum enim copia verborum copiam gignit’, says Cicero, giving a new turn to the old maxim of Cato, rem tene, verba sequentur. If the matter is honourable, the words in which it is expressed will have a natural splendour. (p. 60)
 De Oratore I. 35f.
 De Oratore I. 44
 Quintilian II. xvii. 21
 De Oratore II. 178
 De Oratore II. 189
 De Oratore II. 191, 193. But writing as a moralist in the Tusculans he says oratorem vero irasci minime decet, simulare non dedecet.
 De Oratore II. 195
 Orator 130, 132
 De Oratore III. 52-3
 De Oratore III. 104
 De Oratore III. 124f.