What's the role of the "complaintant" in criminal process?  What about
prosecutorial discretion for criminal offenses - how does it work. 
I'd like to know how
it works when someone reports a possible crime to the police or the
DA.  How much discretion do the police or the DA they have in pursuing
a case once the complaint has come to their attention.  Are they
required to pursue a case after it's been reported?  If they decide
not to, can an interested private party require them to explain the
no-prosecute decision in some forum, and/or bring action to overturn
the no-prosecution decision and compel prosecution?  What criteria are
legally sufficient for a law enforcement agency that decides not to
pursue a case?  What legal rules constrain the exercise of
prosecutorial discretion? Does a DA have unlimited discretion (say to
prosecute the offenses which carry a fine only, and to not process
other offenses/misdemeanors which may carry jail time)?  Presuming
not, what is the nature of that discretion and how is it exercised,
and how can it be legally brought to question?  What ability does a
private parson have to prosecute a criminal case in a court of law,
after the action has been dropped by the DA?

Do the answers differ significantly from jurisdication to
jurisdication (state to state)?

Wow. That's a tall order for the price you have selected. I recommend
you review the pricing guide and adjust your price accordingly in
order to attract a quality answer.

http://answers.google.com/answers/pricing.html

Alternatively, you might select just one or two of the most important
questions and reduce your request to these topics only if you plan to
maintain your current offer.

Regards;
tutuzdad-ga

In most states prosecutors have nearly total discretion in deciding
what cases to prosecute.  I expect that few states would permit a
complainant to force the prosecutor to bring charges.  Some states
permit "private prosecutions" where a complainant can hire a private
attorney to serve as prosecutor.  Those cases are pretty rare.