 The most demanding scene for the actors involved the filming of the  "human chain" employed by the Rangers to cross a treacherous river.  The actors themselves had to do the shots without the benefit of  stunt doubles. The sequence was begun at Payette Lake in Idaho but  had to be completed in the studio tank because the lake was far too  dangerous. For Spencer Tracy (qv), who once complained that the  physical labors required of actors "wouldnt tax an embryo," it was  his most difficult shoot to that point, surpassing even the taxing  ocean scenes of his Oscar