Francis Ford Coppola had written the outline in 1966, but couldn't get financing until Le Parrain (1972) became a success.
The blue Mercedes limousine that Cindy Williams is sitting in near the end of the film was won by Francis Ford Coppola on a bet with Paramount Pictures. Coppola had complained about the station wagon he shared with five other passengers during the filming of Le Parrain (1972). Studio executives told him that if The Godfather had grossed a certain amount, they would spring for a new car. After The Godfather became the highest grossing film of all time, Coppola and George Lucas went to a dealer and picked out the Mercedes, telling the salesman to bill Paramount Pictures.
As Harry refines and re-refines the recording, he interprets what he hears in different ways. In fact, the dialogue was recorded multiple times with different readings to get this effect.