Many people have wondered at the symbolism of Don Corleone's mysteriously holding a kitten in the first scene of the movie -- whether it showed the inner disjunction of a man who could order the cold-blooded death of two young men, and still gently pet a baby cat at the same time.
The true story is this: the kitten (brown mackerel, female) was a stray, who was attracted by the fictional Italian wedding going on over several days (in a nice Staten Island neighborhood, held by th Norton family), with food, long skirts, general chaos and suchlike, and had held up several scenes by distractingly popping up at odd intervals. Brando, however, happened to like cats, and always said "Let me handle this.". He proceeded to pick up and play with the kitten, and soon the kitten was no problem, since she was always where Brando was.
It came time to film the ultra-serious Office Scene. The kitten, now totally inured to cameras, lights and general goings-on, trotted obediently behind Brando. Ever the professional, Brando smiled.
"Let me handle this."
That the kitten was more than satisfied with this, is evident in that several of Brando's lines had to be rerecorded because of the cat's purring. Indeed, this meant that most of the bits and pieces of film where the kitten appeared or could be heard could now be salvaged, since the kitten was now simply a naturalistic detail, since it had been established that the Corleone family kept cats.
That Brando was able to upstage a small, cute, helpless kitten remains one of the greatest acting achievements on film.