Two observations from the source Reddit thread:
Brando was notorius for never learning his lines for movies. Like in Superman, he read his lines off the babies diaper / costume because he refused to read the script before hand.Somehow I always thought that professional actors had impressive memories that I would never be able to match. Now I realize, I could have been an actor. I coulda' been somebody...
"During his long monologue over the body of his wife, for example, Brando's dramatic lifting of his eyes upward is not spontaneous dramatic acting but a search for his next cue."
Brando always claimed that he wanted to say scripted lines as if they were just occurring to him--eventually leading to him being fed lines over an earpiece in Don Juan DeMarco. At least that was his official reason. Maybe he just hated memorizing lines.