According to Brando himself, that study evolved from deeply auditing his characters to not memorizing his lines because … well, that’s not what people do when they talk. They don’t know what they’re going to say in advance, so even the most nuanced reciting of scripted words felt untrue to his ears. In Stefan Kanfer’s book “Somebody: The Reckless Life and Remarkable Career of Marlon Brando” he said the cadence of such rote memorization sounded like “Mary Had A Little Lamb” to his ear. He continued:

“In ordinary life, people seldom know exactly what they’re going to say when they open their mouths and start to express a thought. They’re still thinking, and the fact that they are looking for words shows on their faces. They pause for an instant to find the right word, search their minds to compose a sentence, then express it.”

This caused a little bit of worry for “The Godfather” director Francis Ford Coppola, not so much that Brando would be bad in the role because he was using “idiot cards,” but that he’d inspire the rest of the cast to do the same. Turns out he didn’t have much to worry about on that front as the rest of the cast were more than happy to do it the old-fashioned way and the result is one of the greatest films of all time. So, whether Brando was using naturalism as an excuse for laziness or if he was truly on to something we’ll never know, but I guess the proof is in the pudding.