Experts include accountants, auditors, scientists, engineers, handwriting experts, medical experts such as pathologists etc. In an action for negligence against an auditor, another auditor could be called as an Expert witness. He would testify as to what were proper standards in the profession. He could be asked, and could express an opinion on, whether certain hypothetical work practices were in accordance with acceptable professional standards. Obviously, in this case, he would not be giving eye-witness testimony of facts.
Some witnesses can legally refuse to answer questions.
A witness can refuse to answer a question on the ground that it might incriminate him. This means that if he were to answer the question, it might provide evidence for the bringing of criminal charges against him. If a witness claims this privilege, the judge will decide whether he has to answer the question. A lawyer can refuse to answer a question regarding matters confided to him by a client. He can refuse to produce a client's documents.