Matthew Broderick had no idea that Marlon Brando was going to use the walnuts as a prop in the scene where they discuss his job. While waiting for the scene to be shot, Brando cracked the walnuts just enough so they would be easy to break, and then used them in the scene to help generate a more genuine response from Broderick.
During post-production, Marlon Brando publicly condemned the film and claimed it would be the biggest turkey of all time. This was because Brando asked for an additional $1 million when the shoot was extended an extra week. When the producers refused, Brando threatened to badmouth the film in the press. They still refused and Brando followed through with his threat. The following day, the producers paid Brando the money and Brando publicly praised the film.
According to director Andrew Bergman on the Turner Classic Movies documentary on Marlon Brando, He was having a problem shooting a scene with Brando and as a nervous habit, he began chewing Bazooka bubblegum. Brando asked him for a piece to which Bergman replied "I'll tell you what, you do this scene in one take and I'll give you a piece." They shot the scene in one take and Brando immediately went behind the camera with his hand out. Good to his word, Bergman gave Brando a piece of Bazooka bubblegum.
In addition to the numerous "Godfather" jokes, the film also makes a reference to another Marlon Brando film: at one point, Sabatini mistakenly refers to Clark as "Kent." In the movie Superman (1978), Brando played Clark Kent's father Jor-El. The Gourmet Club Maitre D' is played by Gianni Russo. Russo portrayed Michael Corleone's treacherous brother-in-law Carlo Rizzi in Godfather I and II.
Writer/director Andrew Bergman was intent on persuading the increasingly reclusive actor Marlon Brando to play the role of Mafia chieftain Carmine Sabatini. A few weeks after sending Brando the script, the actor phoned Bergman and invited the director to his home to discuss the movie. Bergman arrived at Brando's Mulholland Drive home and began two days of intensive, non-stop conversations: The director and the actor discussed eastern religion, the economy, politics, philosophy, insects, geology, history, favorite foods, meditation... everything but the movie, the screenplay, or the role of Carmine Sabatini. Finally, after two days of discussions, during a lull in the conversation Brando said, "I don't think I can play this part without referencing some aspect of the Don," referring to his iconic role in The Godfather (1972). Bergman, drawing on his background as a comedy writer, thought for a moment. Then he brightened. "I've got it!" said Bergman, "We'll make Carmine Sabatini the guy 'The Godfather' is based on!" The actor thought Bergman's idea over. "I can live with that," Brando said after a few seconds, "Let's do the picture."
Also according to Penelope Ann Miller on the same documentary, Marlon Brando wore an ear-piece so an assistant off stage could feed him his lines. During a scene together, Miller recalls forgetting her lines, to which Brando asked his assistant over a hidden microphone for the line and he gave them to Miller. She also recalls Brando telling his assistant at one point "No, no, no. That's the wrong scene."
Gianni Russo who plays the Maitre D' at The Gourmet Club played Carlo in "The Godfather" Marlon Brando in the Godfather said of Carlo, "Give him a living, but never a job in the business" As Brando is leaving the Gourmet Club the Maitre D' says "Good Night Boss".
Frank Whaley's character likens the character "Big Leo" to a rather rotund former wrestler named William 'Haystacks' Calhoun. Whaley has no idea who Haystacks was, but writer/director Andrew Bergman (a wrestling fan in his youth) made up the line on the spot for Whaley.