THREE AND FOUR decades ago, children went to the movies on Saturday afternoons clutching in their sticky paws a dime or 15 cents, for which they got one of the great bargains of modern times: double feature, previews of coming attractions, two cartoons and clear proof of immortality.

The proof was furnished in the weekly installment of the adventure serial. Every episode ended with the hero, heroine or both being incontrovertibly and obviously killed: train off a trestle, plane flying into the side of a mountain, 40 million volts surging through a prisoner shackled to a wall in the cave of the spider people. Then, next week, the victims were alive again. The previous Saturday's catastrophe was shown once more, but this time with a new scene inserted: man leaps from speeding train just before crash, or parachutes from plane, or slips his fetters and escapes into the shadows of the spider cave. Nobody ever died.

Resurrection isn't so cheap or simple anymore. Consider the case of Bobby Ewing. He was the good, or at least human, brother of the monster J. R. Ewing, in the fictional television city of "Dallas." Played by the actor Patrick Duffy, he was as necessary a contrast to J. R. as Jekyll was to Hyde. Nevertheless, circumstances required that he be phased out, and in the final episode last season he was involved in an auto accident. He was not killed instantly, however. In fact, his expiration was so prolonged and lachrymose (even J. R. was weeping at his bedside!) that it seemed possible a band of angels might swoop down and carry him away in the final scene.

In the end, however, his death was achieved within the bounds of convention, and in "Dallas" episodes this year, he was shown to have a gravesite at Southfork Ranch, which was occasionally visited by loved ones. Perhaps during one recent visit, a plea somewhat like this was addressed to him: "Bobby, no one can begin to tell you how much we miss you, but I'll try: we've dropped from the No. 2 show to No. 7, and according to the Nielsen figures we've lost 2 1/2 million viewing households. Boy, do we miss you, Bobby."

Perhaps in response, it has been announced that Patrick Duffy will return to the cast of "Dallas" for the season's concluding episode next month. There is speculation that Bobby Ewing's death was too conclusively documented to allow his return and that Mr. Duffy will play the role of a Bobby look-alike. But whoever he is supposed to be, viewers will know he's really Bobby, as surely as they knew many Saturday afternoons ago that the hero didn't really perish in the weekly fireball. Faith in serial immortality has been replaced by a well-founded belief in the ability of the A. C. Nielsen Co. to raise the dead on prime-time soap operas.