When Arnold Schwarzennegger is the star, you don't have to go overboard with the coming-attraction hype; it's enough to show his face. That sums up the trailer philosophy for The Last Action Hero, due in June. You see a scene of an urban-hostage crisis through a mail-slot-shaped peephole as offscreen reporters inform you that the situation is hopeless. A truck charges in, and out steps Schwarzenegger, howitzer in hand. Peering at you through the peephole, he barks, ''Sorry, not ready yet. Come back later,'' and slams the door. We see nary a frame of action. ''We conceived this in the classic teaser tradition,'' says Sid Ganis, president of marketing and distribution for Columbia Pictures. But he adds that the studio settled on the concept as much out of necessity as by design: ''We shot it last October before principal photography had even begun.''