Figuring Out His Voice
Marlon Brando ended up basing his voice off of Frank Costello. He heard recordings of Costello during the Estes Kefauver hearings in 1951. Brando thought it was perfect for Vito, so he went with it.
Making it Difficult
In the scene where Don Vito is carried up the stairs by his associates, Brando actually made it more difficult for them to do that task. He was by no means a small guy, but he still decided to stuff his pockets and wear additional weights so that the guys really struggled.