The law defines an expert as...
An individual with scientific and/or professional training – degrees, courses, practical work - in a field that is well above the level of an average person-on-the-street, and who, as an expert, applies ALL the accepted principles, practices and discipline to his field of knowledge, to reach a conclusion.
(I’m paraphrasing here, the actual wording that currently prevails in California can be found in a case colloquially called “Daubert”, any attorney, or web search, can turn it up.)
This is just the beginning of the expertise required of an expert witness. Explaining your conclusions to your attorney or client is often as difficult as it is to an opposing attorney in a deposition or judge & jury during trial. Court is theatre, not in terms of acting, as in playing a false role, but in terms of using the tools of delivery and persuasion to show the audience that you have done your job correctly. Your clients don’t often understand the specialized world of your expertise and you must educate them, lead them along a path of data, research and logic to arrive at understanding your conclusions.
There is a big cast of protagonists and antagonists -- including your attorney(s), the other attorney(s), their staff and minions -- before you ever get close to a courtroom, judge & jury. Before you get to a courtroom, it is your job to show your team that your procedures, analysis and conclusions are correct. Make no mistake about it, this is sometimes not a happy experience for all. Your conclusions may not be what your client wants to hear.
In being an effective expert witness you will need not only your skills in your field, but tools from psychology and drama in order to get your points across. Did you know that there are many texts out there for expert witnesses on what to wear, how to gesture, when to smile and make eye contact with the jury?