Prosecutors must disclose the informant's identity if it would be relevant or helpful to the defense, or essential to giving the defenant a fair trial. Common examples of this are where the informant was an eyewitness to, or a participant in, any of the offenses that were charged.
An informant can be made to testify at either a pretrial hearing or at a trial. For example, if the informant's identity is ordered disclosed because she was the source of information for a search warrant, and was present and witnessed the illegal acts that resulted in the warrant being issued, the informant might have to testify as a witness at a hearing before trial in which the defense argues the warrant was improperly issued.
If the police or prosecutor don't want to reveal the identify of the informant, either for the informant's safety or so as not to "blow his cover" so the informant can be used in future cases, they'll go to great lengths to avoid having the informant testify. But in the end, it's up to the judge. She'll balance the request not to disclose the informant's identity against the constitutional rights of the accused to present a defense and to have a fair trial.
If you ignore the subpoena and don't appear in court, you may be held in contempt of court. The punishments for contempt include a fine, jail time, or both.
The purpose of perjury statutes is to keep the process of justice free from false testimony.
The best-known form of perjury is falsely answering a question while testifying as a witness at a trial. Before you could be found guilty of perjury, the government must prove beyond a reasonable doubt that you knew what the question meant and that you gave knowingly untruthful and misleading answers in response to the questions. In other words, your knowledge and reasonable understanding as to what was meant by the person asking the question is critical to deciding whether you committed perjury. The government has to prove your statement was false at the time you made it. False testimony that's an honest mistake isn't perjury.
In addition to perjury being a crime in itself, a defendant can receive increased punishment under the USSG for providing a materially false statement to a judge, law enforcement officer, or probation officer.
If you to take an oath to tell the truth, it's important that you do so.
Depending on how long ago the conviction occurred, the witness could be confronted with it in front of the jury. This is called "impeachment." The jury may think less of the witness's testimony knowing that he's got a criminal record. In many states, the judge can instruct the jury that they may consider the witness's prior felony conviction in deciding how much credibility to give the witness's testimony.
If a witness lies and denies the existence of a prior felony conviction, he could be charged with perjury.
In a jury trial, jurors decide the credibility of witnesses, with guidance from the judge in the form of "instructions" provided after all the evidence has been received. The judge instructs the jurors that they're the sole and exclusive judges of the credibility of each witness who testified. If they believe a witness testified falsely, they may decide to believe all of that witness's testimony, only a portion of it, or none at all.
The judge also gives guidance on how jurors should determine the witness's credibility. Jurors are told to carefully judge all of the testimony given, and the circumstances under which each witness has testified. They're advised to consider:
Each juror should make her own judgment or assessment concerning the believability of a witness and how important the witness's testimony is to the case.
If the witness is an informant - someone who provides evidence against someone else for money, or to escape punishment for his or her own misdeeds or crimes, or for other personal reason or advantage - the jurors are instructed to examine and weigh their testimony with greater care than the testimony of a witness who doesn't have such motivations.