Many of the characters are combinations or alterations of characters from the novel, to make for a smaller cast and a less confusing plot:
- Salome and Rosalie Otterbourne, a romance novelist and her daughter, become a singer and her niece/manager.
- Andrew Pennington becomes cousin Andrew Katchadourian.
- Marie Van Schuyler, a socialite and no relation to Linnet, becomes Linnet's godmother who has the ideals of Mr. Ferguson, the aristocrat turned socialist.
- Windlesham, a character mentioned in the first chapter, is combined with Dr. Bessner to become Dr./Lord Windlesham.
- Bouc, a character who appears only in Murder on the Orient Express, takes on elements of Col. Race, while his choice of traveling companion, his mother Euphemia, is reminiscent of the relationship between Tim and Mrs. Allerton.
This movie had been delayed several times. Originally set for a 2019 release, a delay in filming pushed it to 2020. Then, due to the COVID-19 pandemic, it got pushed around several times, until being set for fall 2021. Then on April 3, 2021, it was pushed back to February, 2022, by the Armie Hammer accusations.
Tiffany & Co designed jewelry worn in the film, most prominently Linnet's diamond necklace. The necklace features a replica of the Tiffany Yellow Diamond, one of the largest yellow diamonds ever discovered. First discovered in South Africa in 1877, as of 2022, it has only been worn publicly by four women. At the 1957 Tiffany Ball, it was worn by Mrs. E. Sheldon Whitehouse. It was worn by Audrey Hepburn for Breakfast at Tiffany's (1961) publicity photos. Lady Gaga wore the diamond at the The Oscars (2019). Beyoncé wore the necklace in a campaign with Tiffany in 2021.
This version of Poirot is implied to have been born much later than his literary counterpart, who was in his fifties at the beginning of the Twentieth Century. In this movie, Poirot is shown in the prologue as a young soldier in 1914, making him at least thirty years younger than Book Poirot.
In both this film and Murder on the Orient Express (2017), Poirot shows signs of being obsessive compulsive (Letitia Wright's character even calls him "obsessive" at one point). His need for balance and symmetry is shown when he sends back one of the desserts so that he has an even number and when he straightens the feet of one of the victims. Obsessive-Compulsive Disorder (or OCD) is considered a relatively new discovery, but the term was actually coined in 1877 by German psychiatrist Carl Westphal.
David Suchet: entering the club during the last scene. Suchet played the title character in Poirot (1989).