They both emerged from a period when risk-taking was an essential part of the business and seat-of-the-pants improvisation gave movies their kick and vitality. Spielberg transformed Peter Benchley's best-seller from a brutally efficient page-turner into a primal scream of a suspense film and a rollicking yarn. "Jaws" is entertaining not merely because of Spielberg's almost prehensile touch with suspense, but also because of the games Spielberg plays with expectations on a crowded beach, and the comic hay he makes from the contrasts of his three heroes -- the manly shark-fighter (Robert Shaw), the young, countercultural scientist (Richard Dreyfuss), and the solid husband, father and police chief (Roy Scheider, above). Of course, the ideal demon for a director whose forte is movement is the great white shark: huge, speedy and agile, its forays unpredictable and its viciousness unmatched. Spielberg builds its presence second by second, so that the first full-sized sighting elicits shrieks from his audience that don't seem to end until "The End."