''Suddenly Kennedy clutched his chest and fell flat on the ground. Walking behind him was the dignified Countess Crespi and her small son. Both simply stepped over the President's body - as if he were not there - and continued walking toward the shore.

''Right behind the countess came Jacqueline Kennedy, and she, too, daintily stepped over her husband's body. Behind her was Red Fay, Under Secretary of the Navy and Kennedy's PT boat buddy. Fay stumbled and fell directly on the President's body. Just then, a gush of red surged from the President's mouth, covering his sport shirt.'' A report by The Associated Press dated Sept. 21, 1963, detailed a movie made in Newport but said it was Mr. Fay who lay down on the dock and Kennedy who stumbled over him. Film Isn't in Archives

Mr. Knudsen confirmed the book's account and said the sequence had been shot several times. Reporters observed the action, but none were close, he said; thus, the reference to ''the man with the binoculars.''

He would not say what had become of the movie. No such film is in the archives of the John F. Kennedy library in Boston, said Mary Lee Quinn, an audio-visual curator.

''There were about four other couples there,'' Mr. Knudsen said. ''They thought it would be kind of fun to do it. There was a little dialogue, but I'm not about to repeat it. It was done in confidence, and even though he's dead, it's still in confidence.''