Supernumerary district attorneys shall take the oath of office prescribed by the constitution for judicial officers and shall have and exercise all the duties, power and authority of district attorneys of the judicial circuits or circuit courts and shall, upon request of the Governor, the Chief Justice of the Supreme Court or the Attorney General, conduct investigations, attend any regular, adjourned or special session of any circuit court in any of the judicial circuits of Alabama for the investigation of or the prosecution of any criminal case or the prosecution or defense of any case in which the state is interested. The Governor, any member of the Supreme Court or courts of appeals or the Attorney General may request a supernumerary district attorney to perform duties as those prescribed for assistant attorneys general, either in their respective offices or at such other places within or without the state as such officials may assign him. When on such special assignment at the request or designation of one of the aforementioned officials and performing duties as those prescribed for assistant attorneys general, the supernumerary district attorney shall have all the powers and authority of an assistant attorney general and shall be entitled to the same amount of sick leave and annual leave that accrues to an assistant attorney general; and, while performing such duties at the request of the Attorney General, he shall be designated as a special assistant attorney general.