Casting
Puzo was first to show interest in having Marlon Brando portray Don Vito Corleone by sending a letter to Brando in which he stated Brando was the "only actor who can play the Godfather". Despite Puzo's wishes, the executives at Paramount were against having Brando, partly because of the poor performance of his recent films and also his short temper. Brando was hesitant about getting back into acting, but his secretary Alice Marchak persuaded him to audition. Coppola favored Brando or Laurence Olivier for the role, and Olivier's agent refused the role claiming Olivier was sick; however, Olivier went on to star in Sleuth later that year. Evans pushed for either Carlo Ponti or Ernest Borgnine to receive the part. Bluhdorn proposed Charles Bronson for the role. Others considered were George C. Scott, Richard Conte (who was ultimately cast as Don Barzini), Raf Vallone, Anthony Quinn and Orson Welles. Welles met with Puzo and tried to convince him that he was right for the role.
After months of debate between Coppola and Paramount over Brando, the two finalists for the role were Borgnine and Brando; Paramount president Stanley Jaffe required Brando to perform a screen test. Coppola did not want to offend Brando and stated that he needed to test equipment in order to set up the screen test at Brando's California residence. For make-up, Brando stuck cotton balls in his cheeks, put shoe polish in his hair to darken it, and rolled his collar. Coppola placed Brando's audition tape in the middle of the videos of the audition tapes as the Paramount executives watched them. The executives were impressed with Brando's efforts and allowed Coppola to cast Brando for the role if Brando accepted a lower salary and put up a bond to ensure he would not cause any delays in production. Brando earned $1.6 million from a net participation deal.
From the start of production, Coppola wanted Robert Duvall to play the part of Tom Hagen. After screen testing several other actors, Coppola eventually got his wish and Duvall was awarded the part. Al Martino, a then-famed singer in nightclubs, was notified of the character Johnny Fontane by a friend who read the novel and felt Martino represented the character of Johnny Fontane. Martino then contacted producer Albert S. Ruddy, who gave him the part. Martino was stripped of the part after Coppola became director and then awarded the role to singer Vic Damone. According to Martino, after being stripped of the role, he went to Russell Bufalino, his godfather and a crime boss, who then arranged for news articles to be published that claimed Coppola was unaware of Ruddy's giving Martino the part. Damone eventually dropped the role because he did not want to provoke the mob, in addition to the salary being too low. Ultimately, although Frank Sinatra threatened to bar him from Las Vegas if he took the role, the part of Johnny Fontane was given to Martino.
Coppola cast Diane Keaton for the role of Kay Adams owing to her reputation for being eccentric. Before auditioning, Keaton never read the book, but she decided to read it after she was cast. John Cazale was given the part of Fredo Corleone after Coppola saw him perform in an Off-Broadway production. Gianni Russo was given the role of Carlo Rizzi after he was asked to perform a screen test in which he acted out the fight between Rizzi and Connie.
Nearing the start of filming on March 29, Michael Corleone had yet to be cast. Paramount executives wanted a popular actor, either Warren Beatty, Alain Delon or Robert Redford. In a 2000 interview, Robert Evans recalls this preference when asked, "Was Warren Beatty your first choice for Michael?" He answers: "No, I wanted Alain Delon. He was the type, but he couldn't speak English well. Maybe I did want Warren. I may have thought of Jack [Nicholson] too for it. Jack tells me I did, but I don't remember it. Dustin [Hoffman] desperately wanted to do it." Informed by Evans of the possibility of obtaining the role of Michael Corleone, Delon was already studying the script based on the novel The Godfather and had begun to consider the role with confidence. According to Ali MacGraw, then Evans's wife, he even invited Alain Delon to their estate after mentioning the role to him several times : "Alain [Delon] came to stay at our house when the casting was incomplete. In his back pocket, he always carried the paperback of Le Parrain, which is The Godfather in French. I think he wanted very much to be in it." However, Delon's smooth, polished image didn't fit the intense, brooding character Coppola wanted, and he explains this disagreement over the casting choice by stating: "I realized at one point, Bob Evans wanted a guy that sort of looked like him, and I wanted a guy that sort of looked like me." Delon eventually refused the role and he sets the proposal aside: "I had to learn to speak English with the Italian accent. That did not please me". Subsequently, Evans turned to Ryan O'Neal for the role, owing in part to his recent success in Love Story. Pacino was Coppola's favorite for the role as he could picture him roaming the Sicilian countryside, and wanted an unknown actor who looked like an Italian-American. Paramount executives found Pacino to be too short to play Michael. Many actors also auditioned, including Dustin Hoffman, Martin Sheen, Dean Stockwell, and James Caan. Keaton read with both Caan and Sheen. Burt Reynolds was offered the role of Michael, but Brando threatened to quit if Reynolds was hired. Reynolds declined the role. Jack Nicholson was also offered the role, but declined it as he felt that an Italian-American actor should play the role. Caan was well received by the Paramount executives and was given the part of Michael initially, while the role of Sonny Corleone was awarded to Carmine Caridi. Coppola urged producers Al Ruddy and Evans to see Pacino in Panic in Needle Park. After watching it, they gave Pacino the part, allowing him to have the role of Michael as long as Caan played Sonny. Evans preferred Caan over Caridi because Caan was seven inches shorter than Caridi, which was much closer to Pacino's height. However, Pacino himself couldn't envision himself in the role because he saw Michael Corleone as someone more glamorous, more self-contained — someone more like Alain Delon. He struggled in early screen tests, often forgetting lines and improvising dialogue. Despite this, Pacino took on the role. Despite agreeing to play Michael Corleone, Pacino was contracted to star in MGM's The Gang That Couldn't Shoot Straight, but the two studios agreed on a settlement, and Pacino was signed by Paramount three weeks before shooting began.
Robert De Niro originally was given the part of Paulie Gatto. A spot in The Gang That Couldn't Shoot Straight opened up after Al Pacino quit the project in favor of The Godfather, which led De Niro to audition for the role and leave The Godfather after receiving the part. De Niro was also cast for the role of Sonny Corleone. After De Niro quit, Johnny Martino was given the role of Gatto. Coppola gave several roles in the film to family members. He gave his sister, Talia Shire, the role of Connie Corleone. His daughter Sofia, then an infant, appeared as Michael Francis Rizzi, Connie's and Carlo's newborn son. Carmine Coppola, his father, appeared in the film as an extra playing a piano during a scene. Coppola's wife, mother, and two sons all appeared as extras in the picture. Several smaller roles, like Luca Brasi, were cast after the filming had started. Ruddy's casting choices would earn him more approval from the Italian-American Civil Rights League, with Anthony Colombo reported to have made Ruddy a league captain after the meeting where the film's bit players and extras were chosen.