Luca was definitely scary enough in his physical appearance.
In the book, Luca is described as "short, squat, massive-skulled". Puzo says he is a frightening, intimidating sight. Still, though, we, as readers, have to use a bit of our imagination to help the image along.
But, in the film, to actually see someone of Lenny Montana's size - especially standing next to Brando - gives us an immediate picture that is unmistakable. Michael's description and Luca's halting delivery of his rehearsed speech only underscore this, IMO.
Anyone who has read the book could say that Luca's "backstory" might certainly be enough for an entire movie. And, for me, there's the problem: like the whole Lucy Mancini story, to have included it in GF1 may have detracted a bit from the main story FFC was trying to tell.
Signor V.
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"Orson fall down?"