While the 1990 film adaptation of The Mysterious Affair at Styles is, in many ways, an extremely faithful one, often lifting dialogue verbatim from the text, there are some important differences. For example, in the novel, Christie never really talks about the war in terms of its violence and devastation. All of this must be assumed and inferred. Hastings spends time in a convalescent home, so he is assumed to have been injured, but he never once refers to his injury. Poirot walks with a limp and is a refugee in Essex, but he never talks about the situation in Belgium that drove him from his home. In the novel, the war is a topic of polite conversation—it is gestured to by mention of rationing, fundraising, and sacrifices being made on the homefront, as consumers. But in the film version, the opening scene shows Hastings in the convalescent home, staring at a film depicting the violence of the war. There is film of men hopping over barriers, of tanks exploding, images of death. And the camera zooms in on Hastings' eyes, showing the carnage reflected in his pupils. In the film, Hastings is clearly haunted by his experiences in battle. When Emily has her fit as a result of being poisoned and the commotion wakes the whole house, Hastings is in the middle of a nightmare about the war. He wakes from it with a start. None of these allusions to personal trauma or suggestions of something resembling post-traumatic stress disorder are remotely present in the novel, which suggests that the film is more interested in representing this dimension of Hastings character, and representing the realities of the horrors of war, more than the novel ever intended to do.
A significant omission in the filmed version is that of Dr. Bauerstein's character. He simply doesn't exist. Instead, Emily's family physician, Dr. Watkins, has a passing interest in toxicology and poisons, and Lawrence Hastings is a practicing doctor. In the novel, Lawrence abandons his medical practice to write poetry. With the omission of Bauerstein, Mary Cavendish's character becomes much less complex; instead of being one half of a marriage defined by infidelity and competition, she is portrayed as a more or less doting wife who is betrayed by her husband, but ultimately forgives him for cheating on her with Mrs. Raikes. By omitting Bauerstein and cutting the scene in which Mary tells her backstory to Hastings, her character is considerably flattened in the film. She is also far less observed and emphasized in the film version, which altogether drops Hastings' obsession with her—in the filmed version, Hastings is far more taken with Cynthia Murdock than he is with Mary, whom he barely seems to notice.
In the film, with Bauerstein absent, John becomes a much more aggressive presence and is thus more logically suspected of murdering his mother. His character places far more emphasis on his financial difficulties in the film, and he explicitly lobbies for a larger share of the inheritance to Emily in a fraught scene in her study. Instead of the rather unsuspecting character he is in the novel, the John of the film version is portrayed as one of two major, obvious suspects, next to Alfred.
The most significant difference is the lack of narration in the film, and the fact that the film doesn't even favor Hastings' perspective. What this means is that the film's audience is privy to details that the novel's audience could not possibly know until they are revealed to Hastings. This affects the order in which certain information is presented. For example, in the film, we learn about John's affair with Mrs. Raikes before Alfred Inglethorp attempts to conceal his alibi in favor of double-jeopardy immunity. The early revelation of John's affair with Mrs. Raikes also contributes to him being a stronger suspect in the audience's eyes, because he's seen as a dishonest person earlier on in the story.
Finally, the film further dramatizes the final parlor scene in which Poirot accuses Eve and Alfred of plotting to kill Emily. Whereas in the novel Eve remains silent and Alfred loses his temper and attempts, but fails, to tackle Poirot, the film has them hold each other, declare their love for one another and lack of regret, and condemn Emily Inglethorp in harsh terms. It seems that the film version mimics the standard form of procedural crime revelations on television; the irony is that Agatha Christie popularized "the parlor scene" as a vehicle for revelation and final confrontations in mystery stories. Otherwise, the film rendition of The Mysterious Affair at Styles is exceedingly faithful to its source material in the way that events unfold. The main aspects of the story it misses out on, however, are some of the most interesting aspects of the novel, like Hastings' incessant insecurity over how he's regarded by Poirot, or his equally incessant fawning over Mary Cavendish. The film loses the internal monologue of Hastings, and it unfortunately flattens Mary's character, robbing her of the complexity introduced by both her confession scenes with Hastings and her affair with Bauerstein.
