What is the main difference between an expert witness and other witnesses in court?
Generally, an expert witness is allowed to testify about their own expert opinion and information which is related to that opinion. A fact witness or eye witness, for example, is only able to testify to what they directly observed. Such an individual can typically not testify about what someone else said (also known as hearsay). Psychologists routinely testify about what other people said, their interpretation, and what they believe various pieces of information mean for the individual and the referral questions they were asked to answer. This would all be well outside of the scope of a witness who was not qualified as an expert witness and would be inadmissible.