An arrest simply means a police officer, federal agent, or judge believes probable cause exists that a person committed a crime. Since an arrest is usually made by law enforcement, the arrest often is for a criminal charge that has not been levied or verified by an attorney or judge. Criminal defense lawyers also deal with the substantive issues of the crimes with which their clients are charged. Criminal defense lawyers may also help clients before charges have been filed by a prosecuting attorney: this is done when someone believes he or she is being investigated.
The accused may hire a criminal defense lawyer to help with counsel and representation dealing with police or other investigators, perform his or her own investigation, and at times present exculpatory evidence that negates potential charges by the prosecutor.
A considerable aspect of this work requires the US criminal defense lawyer to have a clear understanding of the United States Constitution. Specifically, the Fourth Amendment protects against unlawful searches and seizures, while the Fifth and Sixth Amendments govern the right to remain silent so one does not become a witness against himself. All of the Amendments to the United States Constitution are guaranteed to the criminal accused against the states via the Fourteenth Amendment. Thus, a criminal defense lawyer must understand each of these rights and protect the accused. Initial work on any criminal case involves a review of the charges and the claimed facts, and analysis of constitutional violations, the prima facie burden of the prosecution, defenses, and affirmative defenses; as well as potential sentence and sentencing issues. Much of the work of a criminal defense attorney then turns to trial preparation. Any proposed offers mad by the State must be compared to the best judgment about the outcome after trial. A criminal defense attorney will usually discuss potential offers with the prosecuting attorney, as an alternative to exercising the defendant's trial right and other rights. Plea agreements, when made, can be characterized as either charge agreements (often involving a less serious charge), sentencing agreements (involving a lesser sentence), or both.