Two questions must be asked before acknowledging someone as a witness:
- Who can give evidence? This question relates to competence.
- Who must give evidence? This question relates to compellability.
Three Important Capacities for a Witness
Perception entails an ability to perceive events as they occur and an ability to differentiate between that which is actually perceived and that which the person may have imagined, been told by others, or otherwise have come to believe.
Recollection refers to the person' capacity to remember his/her actual perceptions of a prior event, and the ability to distinguish those retained perceptions from information provided to the persons from other sources, such as statements made to person by others.
Communication refers to the ability to understand questions and to respond to them in an intelligible fashion.
Every person has a right to everyone else's evidence and a general duty to give what evidence he is capable of giving. Exemptions are exceptional.
Requirement of Oath or Affirmation
Traditionally, it is a promise to one's god to invite the deity to observe testimony in court and punish one who lies.
In this modern secular age, it is common to have people make a solemn affirmation to tell the whole truth and nothing but the truth (secular affirmation) rather than making a promise to a deity.
The law supports the testimony of anyone deemed competent because it is in the interest of the principle of fundamental justice. However, there are several exceptions to this principle for spouses, minors or the mentally disabled. A prior conviction will not automatically discount a witness' testimony, but it may carry weight in deciding whether he is competent. At common law, anyone with an interest in the outcome may be prevented from being a witness because of perjury concerns. However, this rule is no longer in effect in many jurisdictions. Rather, witness credibility will be determined in the cross examination. See also Competency to Testify.
Compelling a Witness to Testify
Anyone who is may be compelled to testify through the use of a subpoena.
There are some exceptions:
- The accused is protected against self-incrimination and has the right to silence.
- Spouses are protected from testifying against each other to protect the marriage.
- Attorney-client privilege: a lawyer cannot be compelled to testify about privileged information received from a client.
- Co-accused: cannot be compelled to testify against one another. Note that this can be an incentive for the prosecution to sever co-accused and try each in separate trials.
Failures to Testify
- The Accused
The court should not comment on or read into the failure of the accused to testify. This protect the accused's right of privilege against self-incrimination. No adverse inferences should be drawn, although in some jurisdictions, appellate courts do draw adverse inferences. If the accused testifies, the prosecution may not ask if the accused has a criminal record.
- Witness Privileges
Marital communications: any witness that is a spouse, even though competent and compellable by the defense, still has a right to refuse questions regarding marital communications.
Attorney-Client Privilege: a lawyer may be a competent and compellable witness, but lawyer-client communication is privileged information and a lawyer must refuse to testify about what a client has told them. Note that priests and doctors have confidence rules too, but in general, these do not extend to the witness box.
Non Self-Incrimination: there is a common law privilege against self-incrimination. Some jurisdictions have a specific right against self-incrimination, while others, such as Canada, force individuals who choose to take the stand to answer, but make the answer inadmissible in subsequent proceedings. Testimony cannot be used in subsequent trials to prove innocence or guilt except in perjury trials.