How many do you have?  (I typically find more than four or five
unwieldy.)  They take, in general, the form of a person (typically the
GM (Game Master)) orchestrating the events that unfold, playing the
`universe' if you will, while the others play characters in that
universe.  Its a load of fun, especially if you're an evil sadist like
myself ... :)
Plenty of Web-Pages out there on the subject.  :)
-- 
   Alexander Williams {thantos@alf.dec.com / zander@photobooks.com}
  Prefect of the 8,000,000th Terran Overlord Government Experimental
      Strike Legion, Primary Transport TOG "Bellatores Inquieti"
======================================================================
   You ride in 250 tons of molecularly aligned crystalline titanium
wedded to a ceramic ablative matrix.  You carry a 200mm Gauss
cannon, two massive 10-gigawatt lasers, two SMLM fire-and-forget
missiles, a Vulcan IV point defense anti-missile system, and a
deadly assortment of other equally lethal weapons.
   Your vehicle is the ultimate product of 4,000 years of armored
warfare.
   Your life expectancy is less than two minutes.
					-- RENEGADE LEGION:
					   CENTURION