Brando took his role quite seriously
"in one scene he uttered two phrases, ``After all, we are not murderers ... in spite of what the undertaker says.'' In between the two phrases, Don Corleone, paused to inhale the fragrance of a rose. It was a brilliant touch. Finding a supporting actor not up to the mark in a crucial scene, Brando, who was supposed to slap him, hit him really hard. The shocked actor reacted with astonishment and rose to the occasion!"
I have to think about the other three. I can see Mikey forgetting and Fredo and Sonny over applying but for different reasons.