In an article published in The New Yorker in 1996 entitled "Why I Wrote The Crucible: An Artist's Answer to Politics," Arthur Miller makes the point that fear, even powerful and paralyzing fear, does not translate well over time.
Miller lived through the frightening McCarthy trials, and he had a lot to be afraid of at that time, as did everyone who lived at the mercy of the House Un-American Committee. The same, of course, is true of those who lived in Hitler's time as well as those who lived three hundred and fifty years before in Salem, Massachusetts.
Miller relates his opinion about fear, first of all::
Fear doesn't travel well; just as it can warp judgment, its absence can diminish memory's truth. What terrifies one generation is likely to bring only a puzzled smile to the next.
Next Miller tells the story to which you refer in your question:
I remember how in 1964, only twenty years after the war, Harold Clurman, the director of Incident at Vichy, showed the cast a film of a Hitler speech, hoping to give them a sense of the Nazi period in which my play took place. They watched as Hitler, facing a vast stadium full of adoring people, went up on his toes in ecstasy, hands clasped under his chin, a sublimely self-gratified grin on his face, his body swivelling rather cutely, and they giggled at his overacting.
Just as the students giggled at the rather ridiculous-looking Hitler twenty years after his fearful reign has become nothing more than a memory, Miller says, anyone who sees footage of Joseph McCarthy is sure to find him a comical figure, as well.
Likewise, films of Senator Joseph McCarthy are rather unsettling...he comes across now as nearly comical, a self-aware performer keeping a straight face as he does his juicy threat-shtick.
The playwrite's point, of course, is that living through such fearful events and times is horrifyingly real; however, with the passing of time, fear diminishes. In the case of Hitler and McCarthy, power and fear existed together; once they were gone, though, the fear has diminished and the threatening men seem more like foolish or ridiculous clowns.