What is an expert? In the legal context, an expert is someone who by training, education and experience understands and can help others understand complex scientific and technical issues and other areas of specialized knowledge. Typically scientists, engineers, medical doctors, accountants, economists, etc. are experts in their fields. In trials and other legal proceedings, experts are the only ones allowed to give opinions from the witness stand. Other witnesses may only testify to facts based on their personal knowledge. The role of the expert in legal proceedings is not limited to testimony. In the litigation context, there are two kinds of experts: consulting experts and testifying experts.
Consulting experts are retained by counsel to assist them in understanding complex issues, developing theories, and selecting and reviewing testifying experts. Consulting experts may assist during depositions and trial. They may be generalists able to understand many more areas than they could testify about. Their work is generally protected from discovery by the "attorney work product doctrine" and their communications, notes, opinions and advice are privileged and cannot generally be seen by the other side in litigation. The consulting expert is not subject to any legal test other than the confidence of the lawyer or party who retains them.
The consulting expert often becomes the lawyer's partner in preparing the case. He or she helps identify the issues, helps select and retain other experts, and helps design and direct tests and other work that needs to be done to evaluate the case. Very often their most important role is helping evaluate the testifying experts work and opinions. This includes both the experts on the same side of the case and on opposite sides of the case. A consulting expert will help prepare the pretrial discovery, including preparing questions that are asked in interrogatories (written questions tendered to the other side) and questions asked in pretrial depositions under oath. The identity of consulting experts can generally be kept from the other side.
A consulting expert may be retained for a very narrow issue or area of expertise or may be retained to assist counsel on all the issues of a case. In my practice, I have used consulting experts in water resources to partner in the preparation of the case from analysis of the initial issues to retaining experts in a variety of fields who can testify on the specific issues. Many experts, particularly those who are more generalists, will excel at the consulting role but might be poor witnesses. Others could be good witnesses but find testifying to be too great a strain.