A fact witness is an individual, sometimes a clinical professional such as a forensic psychologist, who has personal knowledge of events pertaining to the case can testify as to things they have personally observed or witnessed. They may not offer opinions, which are the province of the expert witness, who, in a civil case is either retained by the plaintiff or defense or, more rarely, appointed by the court to make statements about aspects of the case that the professional has personally not observed but in which he or she has specialized training. Ultimately, as a fact witness you are to testify on what you have witnessed, without an opinion.
Expert witnesses offer opinions, unlike a fact witness, that may assist the judge in understanding technical knowledge in order to support their ability to make a sound ruling in a case. An expert witness can be a credentialed specialists in medicine, psychology, economics, and many other other fields. For example, a forensic psychologist can be called upon to testify regarding the mental capacity of an individual who has committed a crime. Another example would be of a forensic psychologist who is retained for a child custody evaluation, also known as a social investigation. Although experts are typically allowed more leeway in testimony than fact witnesses, the content of their testimony may also be carefully examined by the court for validity.
Doctors and other clinical practitioners, such as forensic psychologists, may find that their testimony sometimes straddles the grey area of fact and expert witness. For example, you may be questioned on what you did and what the subject did, like a fact witness, and then asked to state an opinion like an expert witness. For this reason, be sure to consult with the counsel in the case to review your position in the case before you testify.