Brando Was Against Russo
Marlon Brando, being the scary guy that he was, was not thrilled with having Russo on set. He wasn't interested in acting alongside someone who had absolutely no experience. Russo caught wind of what Brando was saying and decided to threaten him. Brando didn't realize it was a real threat and thought that Russo was method acting, so he changed his mind. Whatever works!
The Cheat Sheet
Let's be real: it's hard to memorize lines sometimes! Marlon Brando literally had a cheat sheet set up just off-camera. His co-star, Robert Duvall, would even tape Brando's monologues to his chest. That's friendship!