The Professor's Disregard for His Own People (Situational Irony)
The Professor and his organization represent a national security organization similar to the FBI and the CIA. These organizations are committed to protecting American citizens. When confronted with Roger’s dilemma, however, the Professor states that they are powerless to protect him, and he insinuates that it is better to endanger the life of the innocent Roger than to risk the life of their own agent, who holds the key to important information. While the organization purports to be committed to the well-being of Americans, the Professor is surprisingly callous and uncaring about Roger Thornhill's well-being. Additionally, he is also uncaring about Eve's well-being later in the film, risking her life just for the possibility of gleaning more about Vandamm's operation. When Thornhill confronts him about his ruthlessness, he assures him that "war is hell, even when it's cold."
Kaplan Does Not Exist (Dramatic Irony)
In the only moment in the film in which the audience's point of view diverges from Roger Thornhill's, the viewer learns early on that George Kaplan is in fact a decoy, meant to throw Vandamm and his associates off the trail of an actual double agent working for the U.S. government. Learning this, we know that Thornhill's search for Kaplan is futile, but he has no awareness of this. Had Roger known that George Kaplan did not exist when Eve sent him to the middle of the cornfield, he would not have taken her advice and not risked his life. However, for all Thornhill knows, Kaplan is a real man, and so he continues to search for him up until he ends up in the charge of the Professor. The dramatic irony occurs when the audience knows that Roger's search for George Kaplan is futile, but Thornhill himself has not yet figured out the details of the situation.
Roger Cannot Convince Anyone of His Story (Situational & Dramatic Irony)
From the start, Roger Thornhill is established as a smooth-talking and persuasive businessman. When he dictates notes to his secretary in the first scene, he has an answer to everything and is undeterred by all conflict. When he arrives at the Oak Room, his associate assures the two men with whom he is meeting that he will have them convinced of his pitch by the end of the meeting. Indeed, his professional position as a man working in advertising in New York is emblematic of his considerable powers of persuasion.
A poignant situational irony arises, then, when the charming and consummately persuasive advertising executive cannot convince a single person in his life to believe the story of his abduction. His fall from grace is a rapid one. The unflappable man from the first scene is rendered a buffoon by his evil captors, and an additional dramatic irony arises from the fact that while the audience knows he is telling the truth, no one else believes him.
Roger's Abduction Itself (Dramatic Irony)
The first moment of conflict of the film—when Valerian and Licht mistakenly kidnap Roger, thinking he is George Kaplan—is the first moment of dramatic irony. Before the viewer even has the knowledge that George Kaplan is a decoy, and as such does not exist, we know that Roger Thornhill is not him, and see the moment of mistaken identity as an elaborate misunderstanding. While Valerian and Licht mistake Thornhill's flagging down the hotel attendant as a sign that he is "Kaplan," we know that he is merely flagging down the attendant so that he can get in touch with his mother. Thus, the circuitous plot begins, with the audience aligned with Roger and aware of the truth.
Roger's Mother's Disbelief (Situational & Dramatic Irony)
Roger’s mother's disbelief of his story is ironic in several ways. Utterly skeptical of her son's story, she has the gall to ask Valerian and Licht if they actually plan to kill her son, in the presence of an elevator full of strangers. The laughter that this blunt question incites creates for a tense and terrifying moment of dramatic irony. While the audience knows that Valerian and Licht in fact do intend to kill Roger, everyone in the elevator, including the victim's mother, dismisses this idea as a joke.
In addition, there is situational irony, because one would expect for a mother to have sympathy for her son. However, Mrs. Thornhill seems more skeptical of him than strangers, and even his lawyer. In court, when Roger’s lawyer assures the judge of Roger’s character, his mother scoffs.
North by Northwest Questions and Answers
The Question and Answer section for North by Northwest is a great
resource to ask questions, find answers, and discuss the novel.