For his role as Hobie Doyle, Alden Ehrenreich learned horseback riding, rope tricks, twirling guns and playing the guitar. He has stated twirling the spaghetti, mimicking the lasso, was the hardest part of his role.
Dolph Lundgren has an uncredited (initially much longer and almost entirely deleted) cameo as the submarine captain. Lundgren said that he was very honored to play the part, having never expected to be asked for a film directed by Joel Coen and Ethan Coen.
The fictional film studio "Capitol Pictures" previously appeared in Barton Fink (1991), another Los Angeles period film from the Coen Brothers. Eddie Mannix's secretary mentions that his meeting with the four religious leaders takes place in the "Wallace Beery Conference Room." In the previous film, Barton Fink was a screenwriter who was writing a script for Capitol Pictures, for a wrestling movie starring real-life actor Wallace Beery.
The first film that cinematographer Roger Deakins has shot for directors Joel Coen and Ethan Coen, and on 35mm film, since True Grit (2010). He had previously become an advocate of shooting digitally, and used Arri Alexa cameras on his next five films, but switched back to film for this because the Coens dislike digital cameras and thought that shooting on film suited a period piece set in 1950s Hollywood.
For his role as a Swedish director, Christopher Lambert worked with a dialogue coach who told the actor that no one knows what a Swedish accent really sounds like. Instead, Lambert was instructed to blend his French accent with a German accent. Obviously, anyone who has heard any of the four members of the Swedish music group ABBA speak, will know what a Swedish accent sounds like, amongst other famous Swedes.
The Lockheed recruiter tells Eddie the Bikini Atoll test occurred three weeks earlier, which places the movie in March 1954, but the "copyright notice" shown in one of the pictures includes the Roman numeral for 1951.
There actually was a Capitol Pictures (aka Capitol Film Exchange) in early Hollywood, although not in the 1950s (it lasted from the mid-'20s to the early '30s). Unlike this film's "Capitol Pictures", which was a major studio comparable to MGM, the real Capitol was a "Poverty Row" studio that churned out low-budget "B" pictures.
The photo that the Lockheed recruiter shows Eddie at his lunch meeting is correctly stated by the character to be a picture of the Bikini Atoll nuclear test. However, that exact photo was taken of an atomic (not thermonuclear) test called "Baker" from Operation Crossroads on July 1946, where the bomb was suspended underneath a landing craft and surrounded by 10 other ships.
Jonah Hill had previously stated in Superbad (2007) he did not have the luxury of the Coen Brothers, his favorite directors, directing the pornographic videos he viewed. This is his first time working with the Coen brothers.
Fifth film featuring Ralph Fiennes and Michael Gambon. They previously appeared together in four of the Harry Potter films. However, in this film, Michael Gambon doesn't appear on screen; he only narrates.
Scarlett Johansson's character plays a mermaid in a movie. Johansson previously played the voice of Mindy the Mermaid in "The Spongebob Squarepants Movie" (2004). Also, this movie, "Hail, Caesar," has Clancy Brown in the cast. Brown plays the voice of Spongebob's boss, Eugene Krabs, in the series and movies.
Tilda Swinton portrays twin sister celebrity newspaper columnists Thora and Thessaly Thacker. This may be a tribute to newspaper advice columnists Ann Landers and her twin sister, Dear Abby, both writing under pen names.
"Hail, Caesar" stars three actors from the Marvel Cinematic Universe (MCU) who appear in "Avengers Endgame": Josh Brolin (Eddie Mannix): Thanos; Scarlett Johansson (DeeAnnaMoran): Natasha Romanoff/Black Widow; Tilda Swinton (Thora & Thessaly Thacker): The Ancient One.
The name of the movie studio in Blake Edwards' earlier film S.O.B. (1981) was called "Capitol Studios" it being a similar name to the Coen Brothers' fictional film studio of "Capitol Pictures" in both Hail, Caesar! (2016) and Barton Fink (1991).
The trivia items below may give away important plot points.
The big secret about Baird Whitlock, which the Thacker sisters each threaten to reveal to their respective gossip columns, is likely based on a long-established rumor regarding Gone with the Wind (1939). Reportedly, Clark Gable insisted that original director George Cukor be replaced because Gable had, years earlier, engaged in a sexual liaison with the influential director as a way to further his own career.
"Hail, Caesar!", the film-within-the-film, pays several tributes to Ben-Hur (1959), including the matching subtitle "A Tale of the Christ" and a scene in which a Roman general portrayed by Baird Whitlock (George Clooney) becomes speechless upon encountering Jesus Christ.
In this film, the briefcase with the ransom money is ultimately lost at sea. Another film by the Coens, Fargo (1996), also features a briefcase with ransom money being lost. The Big Lebowski (1998) also features a briefcase with what was assumed to contain ransom money being lost.
Scarlett Johansson's character discovers that she is pregnant as her film is about to go into production. In real life, Johansson became pregnant a few weeks before Avengers: Age of Ultron (2015) started production.