Plaintiffs and defendants often engage expert witnesses after consulting with their counsel. The role of such witnesses is to assist legal proceedings by testifying in the court about areas where they can give their professional opinions.

Accountants, doctors, architects and many other types of professionals may all be asked to appear as expert witnesses, depending on the nature of the proceedings.

The selection of an appropriate expert witness is sometimes very important to the success or failure of the proceedings. For example, if an accountant engaged as an expert witness in legal proceedings involving fraud can explain certain complex accounting treatments to the jury in layman's terms, he or she will certainly be much better than one who explains them in a highly technical way.

Plaintiffs and defendants are both free to engage their own expert witnesses. However, an expert witness's credibility and his or her evidence may be challenged by their opponent's counsel. Furthermore, the Court is not bound to accept all the evidence presented by an expert witness.

In addition, the courts have established guidelines about various duties and responsibilities that expert witnesses have to observe in different types of proceedings. The following extract from the judgement in "National Justice Cia Naviera SA v. Prudential Assurance Co. Ltd. (The Ikarian Reefer) [1993] 2 Lloyd's Rep.68 at p. 81" summarises these clearly:

1. Expert evidence presented to the Court should be, and should be seen to be, the independent product of the expert uninfluenced as to form or content by the exigencies of litigation (Whitehouse v. Jordan, [1981] 1 W.L.R. 246 at p. 256, per Lord Wilberforce).
2. An expert witness should provide independent assistance to the Court by way of objective unbiased opinion in relation to matters within his expertise (see Pollville Ltd. v. Commercial Union Assurance Co., Plc., [1987] 1 Lloyd's Rep. 379 at p. 386 per Mr Justice Garland and Re J, [1990] F.C.R. 193 per Mr. Justice Cazalet). An expert witness in the High Court should never assume the role of an advocate.
3. An expert witness should state the facts or assumption upon which his opinion is based. He should not omit to consider material facts which could detract from his concluded opinion (Re J sup.).
4. An expert witness should make it clear when a particular question or issue falls outside his expertise.
5. If an expert's opinion is not properly researched because he considers that insufficient data is available, then this must be stated with an indication that the opinion is no more than a provisional one (Re J sup.). In cases where an expert witness who has prepared a report could not assert that the report contained the truth, the whole truth and nothing but the truth without some qualification, that qualification should be stated in the report (Derby & Co. Ltd. and Others v. Weldon and Others, The Times, Nov. 9, 1990 per Lord Justice Staughton).
6. If, after exchange of reports, an expert witness changes his view on a material matter having read the other side's expert's report or for any other reason, such change of view should be communicated (through legal representatives) to the other side without delay and when appropriate to the Court.
7. Where expert evidence refers to photographs, plans, calculations, analyses, measurements, survey reports or other similar documents, these must be provided to the opposite party at the same time as the exchange of reports (see 15.5 of the Guide to Commercial Court Practice)."

In a nutshell, extremely high standards of performance are expected from expert witnesses. It is therefore advisable to give careful consideration to engaging them, and proper legal advice should certainly be sought throughout the entire process.