“The qualities of a good prosecutor are . . . [elusive and . . . impossible to define]. …
The prosecutor has more control over life, liberty, and reputation than any other person in America. His discretion is tremendous. He can have citizens investigated and, if he is that kind of person, he can have this done to the tune of public statements and veiled or unveiled intimations. Or the prosecutor may choose a more subtle course and simply have a citizen’s friends interviewed. The prosecutor can order arrests, present cases to the grand jury in secret session, and on the basis of his one-sided presentation of the facts, can cause the citizen to be indicted and held for trial. He may dismiss the case before trial, in which case the defense never has a chance to be heard. Or he may go on with a public trial. If he obtains a conviction, the prosecutor can still make recommendations as to sentence, as to whether the prisoner should get probation or a suspended sentence, and after he is put away, as to whether he is a fit subject for parole. While the prosecutor at his best is one of the most beneficent forces in our society, when he acts from malice or other base motives, he is one of the worst. …
Nothing better can come out of this meeting of law enforcement officers than a rededication to the spirit of fair play and decency that should animate the federal prosecutor. Your positions are of such independence and importance that while you are being diligent, strict, and vigorous in law enforcement you can also afford to be just. Although the government technically loses its case, it has really won if justice has been done. . . .
There is a most important reason why the prosecutor should have, as nearly as possible, a detached and impartial view of all groups in his community. Law enforcement is not automatic. It isn’t blind. One of the greatest difficulties of the position of prosecutor is that he must pick his cases because no prosecutor can even investigate all of the cases in which he receives complaints. If the department of justice were to make even a pretense of reaching every probable violation of federal law, ten times its present staff would be inadequate. We know that no local police force can strictly enforce the traffic laws, or it would arrest half the driving population on any given morning. What every prosecutor is practically required to do is to select the cases for prosecution and to select those in which the offense is the most flagrant, the public harm the greatest, and the proof the most certain.
… A sensitiveness to fair play and sportsmanship is perhaps the best protection against the abuse of power, and the citizen’s safety lies in the prosecutor who tempers zeal with human kindness, who seeks truth and not victims, who serves the law and not factional purposes, and who approaches his task with humility” 
State Prosecuting Attorneys
Federal Prosecuting Attorneys
Selection and Qualifications of Prosecutors
diversions (pre-trial contracts between the government and the defendant which divert cases out of the system), work with law enforcement officers from other states who seek to extradite offenders, prepare the accusation to present to grand jury, call witnesses and present a prima facia case (present enough evidence which, when unrebutted by the defendant, shows that the defendant committed the crime) at a preliminary hearing, represent that state at arraignments and status conferences, conduct the trial, and, upon conviction, make sentencing recommendations while representing the state at the sentencing hearing.