Cross-examination has rightly been described as the most powerful engine for the discovery of truth known to man – in the courtroom context of course. But even cross-examination has bounds, which if exceeded makes it subject to abuse. The Supreme Court of Canada has described cross-examination in poetic terms:
Cross-examination may often be futile and sometimes prove fatal, but it remains nonetheless a faithful friend in the pursuit of justice and an indispensable ally in the search for truth. At times, there will be no other way to expose falsehood, to rectify error, to correct distortion or to elicit vital information that would otherwise remain forever concealed.
The Court said this in a case called Lyttle. The Court also made it clear that trial lawyers can cross-examine on factual matters which they may not be able to prove directly, provided they have a good faith basis for the factual premise of the questions. The good faith requirement applies in criminal cases, and in civil cases which involve claims for damages for personal injury – indeed good faith is required of cross-examination in all branches of the law.
Boundaries are set so that cross-examination is not abused. Cross-examination must be, in one degree or another, relevant. Lawyers also must not turn to “harassment, misrepresentation, repetitiousness or, more generally, from putting questions whose prejudicial effect outweighs their probative value”: Lyttle.
Or as the Code of Professional Conduct puts it at Rule 5.1-2, the lawyer must not unnecessarily abuse, hector or harass a witness, must not knowingly assert a fact as true when its truth cannot reasonably be supported by the evidence, and must not make suggestions to a witness recklessly or knowing them to be false.
These are not only excellent suggestions for professional conduct, but are prohibitions which are binding on lawyers who do trial work or employ cross-examination in any context.