Originally from Take the Witness: Cross-Examination in International Arbitration, Second Edition 

Advocates use a variety of skills to ascertain and test the quality of evidence in a dispute. Among those skills, analysis and reason are frequently aided by cross-examination. While it has sprung principally from the common law, cross-examination has gradually taken its place in international arbitration as a valuable tool in weighing and understanding the evidence of witnesses. Just as is the case with other types of advocacy resources, the art of cross-examination can be practiced in different ways. No one approach is necessarily the best or the only way to do it.

Like other practices that are followed in presenting or defending a case, cross-examination must have a purpose. Whether that purpose is establishing the unreliability of a particular witness or simply exposing a narrative unique to such a witness will depend on the individual case and, to some extent, on the advocate’s judgment.  All of which leads to a specific question, when is it advisable to be friendly with a witness and when do you impeach a witness?

First, a word about terminology:  To impeach strictly means to challenge the veracity of a witness or the authority of a document.  However, impeachment often has a broader meaning, referring more to a softer sort of cross-examination which is intended to raise a doubt about the reliability of a witness or the weight to be given to his evidence, not because he is necessarily a liar, but because he has a poor or inconsistent memory, or is contradicted by documents or another, more reliable witness.  Here, I use the term in this broader sense.  It may also include questioning designed to develop facts or opinions unique to a particular witness.