Generally speaking, criminal cases don't usually go to a full-fledged trial. There are many phases to preparing criminal defense on any charge, beginning with gaining a clear understanding of the facts from the defendant's perspective. Investigation, police report review, getting to know a defendant's personality, background and history with law enforcement and establishing the credibility of witnesses are all relevant steps in addressing allegations of criminal behavior.
On those occasions when it's necessary to proceed to trial, it may be helpful to present witnesses for the defense. The New Hampshire Rules of Evidence provide criteria for witnesses who will appear in court. Generally, every person is competent to provide witness testimony unless otherwise specifically prohibited by statute or rules. A witness acceptable to the court must have the capacity to observe, remember, narrate and understand his or her duty to tell the truth.
For a witness to testify, evidence must be introduced to the court sufficient to support that he or she has personal knowledge of the matter. This can be through the witness's own testimony if it fits the expert witness rule provisions. The witness will be required to declare under oath his or her testimony will be truthful. A member of the jury may not testify as a witness in a case on which the juror sits.
Any party to the proceeding may attack or support by evidence the credibility of any witness, but only to character for truthfulness or untruthfulness, and only if it has been attacked by opinion or reputation evidence or otherwise. The court has discretion to allow or disallow specific instances of a witness's conduct for this purpose. Evidence of beliefs or opinions regarding religion isn't admissible to show they impair or enhance the witness's credibility.
The court has control over interrogating witnesses and presenting evidence to protect them from harassment, embarrassment, wasting of time or disruption in the goal of ascertaining the truth. The court may not question a witness directly except impartially under extraordinary circumstances. In criminal cases, the court will exclude witnesses, other than a victim of the crime, to keep them from hearing other witness testimony.
Source: New Hampshire Rules of Evidence, "Article VI. Witnesses," accessed May. 27, 2015