Before a case goes to trial, both sides want to know what the witnesses will say. If a witness is friendly, the parties or their attorneys may talk to him and ask what he knows about the events that are relevant to the case. However, if the witness changes his mind before trial, the party questioning the witness at trial can do only one of two things. If the witness in court admits making the contradictory out-of-court statement, the trier of fact (judge or jury) might disregard all of the witness's testimony. However, if the witness does not admit making the contradictory statement, then the party questioning him might have to call another witness who heard the earlier statement. This creates a "swearing match" in which the trier of fact has to decide which witness is more credible.

A more effective way of avoiding such surprises and making sure that a witness does not change his story at trial is to get the first statement in writing and under oath.

Affidavits

An affidavit is merely a written record of a witness's out-of-court statement that is signed and sworn to by the witness in front of a notary public or other official authorized to administer oaths. A written statement that is not signed under oath (or "notarized") may be used in court under certain circumstances. It is a crime in most jurisdictions to lie under oath. Thus, witness affidavits may be used to impress upon a witness the importance of telling the truth in his out-of-court statement. The witness can then be asked whether he was lying when he made the first sworn statement out of court or when he gave the contradictory sworn testimony in court.

Form of Affidavit

An affidavit must identify the maker, or witness, along with the date and place that it is made. It should state that it is made under oath and should include the maker's signature along with that of the notary public or official administering the oath, who should also witness the maker's signature. The notary or official should also state his title and whether his authority (or "commission") expires on a certain date or is permanent. In order to be admissible in court, most witness statements must be made on personal, first-hand information. However, witnesses who have expertise in some technical or scientific field of work or study may make affidavits that render opinions on matters that fall within their expertise.

Depositions

A deposition is an opportunity for a lawyer to ask questions of a witness under oath before trial and to have those questions and the witness's answers recorded. This may be done for either of two reasons:
  • If an attorney's client or a favorable witness is dying or lives far from where the trial will take place, an attorney may want to preserve that testimony, which would otherwise be unavailable at the time of trial.
  • an attorney may want to discover what a witness favorable to the other side will say in response to certain questions. This is called an adverse deposition.

In either case, the questioning attorney must give notice of the time, date and place of the deposition to opposing parties in order to be able to use the transcript or recording of the deposition in place of the witness at trial. If a witness will not voluntarily appear for a deposition, the court may allow the parties to subpoena the witness and compel him to attend.

Before a transcript or recording is used at trial, the witness will usually be given an opportunity to review it and make corrections. Also, the attorneys for either side may make objections to the form of the questions, the competence of the witness, or the legality of the procedure. The court may review and rule on those objections before trial. In most jurisdictions, the final transcript or recording can then be used for any purpose at trial, such as refreshing the witness's recollection, impeaching the witness's credibility, or as direct testimony of some matter that the witness might forget or omit.