M. Tullius Cicero Pro L. Flacco 11

[11] How ready to give evidence was Publius Septimius; how angry was he about some former trial, and about his steward: yet he hesitated; yet his scrupulousness was at times at variance with his anger. Marcus Caelius was an enemy to Flaccus, because, as Flaccus had thought it wrong for one publican to decide on the case of another publican, though the case was ever so evident he had been removed from the list of judges. And yet he restrained himself; and brought nothing into the court which could injure Flaccus except his own inclination to do so. 5
  If these men had been Greeks, and if our habits and principles had not had more influence than indignation and hostility, they all would have said that they had been plundered, and harassed, and stripped of their fortunes. When a Greek witness comes forward with a desire to injure a man, he does not think of the words of his oath, but of what he can say to injure him. He thinks it a most shameful thing to be defeated, to be detected, to allow his enemy's innocence to be proved. That is the contest for which he prepares himself; he cares for nothing beyond. Therefore, it is not the best men, nor the wisest, but the most impudent and talkative men who are selected as witnesses.