Many cases turn on the testimony of an expert witness. The most persuasive experts appear impartial and well qualified. However, some experts seem biased. They are only hired by certain types of parties (either defence or plaintiff), which makes them reek with the perfume of “eau de hired gun”. This stench seeps into their testimony and undermines their creditability and reliability. Justice Sharpe writing for the Court of Appeal in Moore v Getahun, 2015 ONCA 55 outlines key considerations when using experts.
Justice Sharpe states:
 Expert evidence is a significant and controversial feature of modern civil litigation. It constitutes an exception to the rule that witnesses may only testify as to facts, not opinions, and that it is the exclusive prerogative of the trier of fact to draw inferences from proven facts. The expert evidence exception operates where specialized knowledge is required to determine the implications of the bare facts and where the trier of fact is not competent to draw the necessary inferences unaided: R. v. Mohan,  2 S.C.R. 9, at p. 23; R. v. Abbey, 2009 ONCA 624, 246 C.C.C. (3d) 301, at para. 94.
 Expert evidence has become more significant with the explosion of scientific knowledge and technical innovation. Many cases have been described as a “battle of experts”. Medical negligence cases are a prime example. The trier of fact requires the assistance of expert witnesses to decide issues pertaining to the standard of care, causation and prognosis.
 The use of expert evidence poses difficult issues that have been the focus of consideration in civil justice reform. How do we control the added costs associated with the explosion of expert witnesses? How do we ensure that a party has a fair opportunity to challenge an adverse expert witness? How do we ensure that expert witnesses offer an unbiased scientific or technical opinion based upon their training and expertise, rather than act as “hired guns” who present unbalanced opinions unduly favouring the party that retains them?…
 While some judges have expressed concern that the impartiality of expert evidence may be tainted by discussions with counsel (see the cases cited below, at para. 72), banning undocumented discussions between counsel and expert witnesses or mandating disclosure of all written communications is unsupported by and contrary to existing authority: see Maras v. Seemore Entertainment Ltd., 2014 BCSC 1109,  B.C.W.L.D. 4470, at para. 90 (“[c]ounsel have a role in assisting experts to provide a report that satisfies the criteria of admissibility”); Surrey Credit Union v. Willson (1990), 45 B.C.L.R. (2d) 310 (S.C.), at para. 25 (“[t]here can be no criticism of counsel assisting an expert witness in the preparation of giving evidence”). In Medimmune Ltd. v. Novartis Pharmaceuticals UK Ltd. & Anor,  EWHC 1669 (Pat.), the court pointed out, at para. 110, that in some highly technical areas such as patent law, expert witnesses “require a high level of instruction by the lawyers” which may necessitate “a high degree of consultation” involving “an iterative process through a number of drafts.”…
 If an expert’s report has not been entered into evidence as an exhibit, it has no evidentiary value, even if provided to the trial judge as an aide memoire. Inconsistencies between the viva voce evidence of an expert witness and his or her written report are the proper subject of cross-examination. However, if the expert witness was not cross-examined as to an inconsistency between his or her viva voce evidence and the contents of their report, it is not open to a trial judge to place any weight in assessing the expert’s credibility on this perceived inconsistency. This is not a mere technicality but rather a matter of trial fairness. The expert witness is entitled to be openly confronted with what may appear to be contradictions so that he or she has the opportunity to explain or clarify the apparent inconsistencies.
Moore v Getahun, 2015 ONCA 55: http://bit.ly/1uFzDfY