Quintilian, and J.S. Watson. Institutes of Oratory.Edited by Lee Honeycutt, 2010. Kindle Electronic Edition. Book V Chapter 7.
Quintilian discusses written and testimonial witness in book five chapter seven. Because written testimony is presented without the witness present it tends to bear less weight than testimony of someone who took the effort to be present (Quintilian V.7.2). When the witness is present it is possible to discredit either the witness or the facts of the testimony (Quintilian V.7.3ff). Certainly an attack on the credibility of a witness can be turned into a positive statement for the opposing cause (Quintilian V.7.6).
It is necessary to this end that the orator know all the details of a case (Quintilian V.7.7) as well as the disposition of the judge. The orator must also know the nature and likely answers of the witness (Quintilian V.7.10). The results of an unknown witness giving unexpected testimony may be unpleasant. The orator does need to take care in questioning witnesses. Different people will respond in more or less credible manners when questioned in different ways (Quintilian V.7.26). Ideally, the questioner can produce any desired answer from a witness, though the skill is not taught well in schools (Quintilian V.7.27-28).
Testimonial evidence will always have weaknesses. The orator must be aware of those weaknesses and ways to deal with them (Quintilian V.7.34).