And the 'differing agendas' idea is really important. For an Alien rpg, having a bunch of players who are all "we want to be a squad of marines who shoot aliens" would, narratively, be death on wheels.
EDIT: to expand on that, of course more experienced rpgers will have their own agendas and things. But in a freeform game, having a conflicting agenda can make you 'the jerk who got Bob's character killed', whereas in this cinematic idea, you're simply playing your part to the best of your ability, because you're supposed to.