Witness is one who has first hand knowledge about an event or a fact in issue or of the making of a document which is material to a case. A subpoena directs a person to appear as witness in a trial. It is the instrument used to compel the attendance and testimony of witness. In most jurisdictions it is mandatory on part of the witness called for to comply and testify the truth, failing which he may be liable to the penalty of perjury.
Mostly evidence must be tendered in trial through a witness who has affirmed to tell the truth from his knowledge of relevant facts in issue. Even documentary evidence has to be sponsored through a witness associated with the particular document. The rules of evidence are concerned with the types of evidence that may be sought from witnesses and the manner in which direct examination and cross examination of the witnesses is done. Witness testimony is desirable and far more reliable than circumstantial evidence.
Anybody is deemed to be qualified to be a witness in court or legal proceedings and everybody is obliged to give evidence if his testimony is sought for.
However, legal rules sometimes disqualify persons from being witnesses in certain cases. Similarly legal rules sometimes exempt persons from the obligation of giving testimony in witness action.
There are legal rules on witness competence that disqualifies certain persons to be witness in particular cases. For instance, neither a juror nor a judge is eligible to be a witness in a trial in which they are acting in such capacity.
Privilege rule provides the holder of the privilege with the right to dissuade a witness from testifying. These privileges are designed to protect confidential or private communications, which are socially valued, between persons sharing different relationships. The perception is that disclosure might be a serious setback to such relationships. List of recognized privileges varies across jurisdictions.
Some instances of the common privileges recognized in different jurisdictions are attorney-client privilege, doctor- patient privilege, clergy- penitent privilege, without prejudice privilege, marital secrets privilege, adverse spousal testimony privilege, state secrets privilege, etc.