Institutio Oratoria

For the opposing counsel have a way of making a witness lose his head or of leading him into some trap; and once a witness trips, he does more harm to his own side than he would have done good, had he retained his composure and presence of mind. The advocate must therefore put his witnesses through their paces thoroughly in private before they appear in court and must test them by a variety of questions such as may well be put to them by his opponent. The result will be that they will not contradict themselves or, if they do make some slip, can be set upon their feet again by a timely question from the advocate who produces them.

Still, even in the case of witnesses whose evidence is consistent, we must be on our guard against treachery. For such witnesses are often put up by one's opponent and, after promising to say everything that will help our case, give answers of exactly the opposite character and carry more weight by the admission of facts which tell against us than they would have done had they disproved them.