Sir Kenneth Branagh was aware of the over-extravagance of his character's mustache, but he justified it by the fact that there were fifteen quotes in the original novel mentioning the mustache.
Branagh really walked atop an actual train for the scene where Poirot does the same, and despite the addition of a safety wire, he found it properly terrifying "for this [then] fifty-six-year-old actor."
Johnny Depp asked director Branagh to apologize to Derek Jacobi on his behalf after filming their scene together "because I had to shout at him, I don't want to shout at Derek Jacobi."
The Fragilite cake scene, the exchange between Hercule Poirot and Edward Ratchett, was filmed over a day and a half. In addition to improvisation, the scene also required numerous bites of cake. According to Sir Kenneth Branagh, Johnny Depp "is absolutely unafraid of a world most actors live in fear of, which is eating on-screen."
Much of the cast and extras are made up of Sir Kenneth Branagh's closest friends and acquaintances, with many of them having either been directed by Branagh in his previous projects, co-starred with Branagh in a movie or theatre production, and/or have been a member of Branagh's eponymous theatre company.