Adaptation Displacement: The movie is a classic, while the novel is overshadowed both by the movie and Highsmith's later works, particularly the Ripliad.
Adult Fear: The merry-go-round scene. Imagine your kids riding it while two men are in a scuffle, one them a murderer, and then the ride crashes.
Crowning Moment of Funny: while Bruno is stalking Miriam, a little kid walks up to him pointing a toy gun and says "Bang, bang!" Bruno just gives him a blank look for a moment. As the kid leaves, Bruno pops his balloon with his cigarette and the kid gives him this unbelieving look.
Foe Yay: Bruno's plan would have worked - had he not been so interested in Guy.
Harsher in Hindsight: Robert Walker, who played Bruno, suffered from mental and emotional problems in Real Life. Less than two months after this film's premiere, he had an emotional outburst following a bout of drinking. His housekeeper summoned Walker's psychiatrist, who administered an injection of sodium amytal to try and calm him down; the drug interacted with the alcohol in his system to cause a severe (and ultimately fatal) reaction.
Idiot Ball: A fair amount of the drama could have been easily avoided if Guy had just gone to the police after finding out Bruno killed his ex-wife. If Bruno accused him of conspiracy, Guy could mount an easy defense since there was no exchange of money, no evidence of coercion, the two men had never met before and Bruno's father could've easily testified to his son's general instability, which he probably would have done because he believed Bruno needed help. In addition he was more-or-less the son-in-law of a Senator, which would have probably given him access to a more than competent defense attorney.
What's more, the police could've easily verified Guy's alibi for the night of the murder. Even if Guy's witness didn't remember him, the platform/train attendants who presumably took his bags at New York and then Washington could have identified him and debunked the theory that he got on at Baltimore.