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.
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."
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."
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."
Big screens with animations of moving landscapes were installed on the set to create the visual effect of a moving train. Josh Gad experienced some motion sickness during filming due to this.