Institutio Oratoria

If this fails, we must content ourselves with making it clear that he is reluctant to tell what he knows, and lead him with a view to tripping him up on some point or other, even though it be irrelevant to the case; we must also keep him in the witness-box for an unusual length of time, so that by saying everything that can be said and more than is necessary on behalf of the accused, he may be rendered suspect to the judge. Thus he will do the accused no less harm than if he had told the truth against him.

But if (to proceed to our second supposition) the advocate does not know what the intentions of the witness may be, he must advance gradually inch by inch and sound him by examination and lead him step by step to the particular reply which it is desired to elicit.