David's Body (Dramatic Irony)
The entire film is based around an instance of dramatic irony, in that the audience knows from the beginning that David has been killed and his body stored in the chest that will serve as the dinner table, but all of the party guests have no idea and are waiting for David to arrive. The film is premised on the tension that arises from this dramatic irony. Characters make polite conversation, everything goes on as planned, but little do they know that there is a body inside the chest. The audience is privy to some terrifying information, and must watch as the characters contend with the mystery almost completely unawares.
The "Perfect Murder" (Situational Irony)
Another potent irony in the film is that Brandon and Phillip are so confident that they will pull off the perfect murder without a hitch. Brandon's confidence is so immense that he drops oblique hints throughout the dinner party that David is dead, much to his guests' confusion. After such displays of his superiority over his peers, the party guests leave without even suspecting that Brandon has anything to do with David's disappearance. However, Cadell is on to the two murderers, after noting Phillip's paranoid anxiety and mistakenly picking up a hat with David Kentley's initials on the way out. While the entire premise of the film is built around Brandon's expectation that he will be able to pull off the perfect artful murder, they are ultimately discovered by the crafty and suspicious Rupert Cadell (and indeed it is partly Brandon's very confidence that they won't be found out which, in the end, leads them to be discovered). In a climactic moment, Cadell discovers David's body and violently reprimands his former students. Brandon's entire plan is unraveled, and his "perfect murder" is rendered imperfect.
The Rope as Book Holder (Dramatic Irony)
Paralleling the dramatic irony about David's body being stored in the buffet table at the dinner party is Brandon's use of the rope with which he killed David to tie up the bundle of books he gives to Mr. Kentley. As if secretly storing the guest of honor's body in the dinner table was not enough, Brandon dreams up an even more pointed and diabolical prank. The irony is two-fold: the viewer knows that the rope is a murder weapon, but more particularly that the murder weapon is now unknowingly in the possession of the victim's father. Mr. Kentley has no idea that the rope was used to kill someone, let alone that it was used to kill his son.
Mr. Cadell's Cigarette Case (dramatic irony)
When Cadell comes back to retrieve his cigarette case, the viewer sees him pull it out of his inner jacket pocket and place it—unseen by Brandon or Phillip—on top of the chest where they are hiding David’s body. While the two murderers have no idea that he did not in fact leave his cigarette case behind, and is only using it as an excuse to come back and find out more about the suspicious events of the night, the audience sees that he is still in possession of his cigarette case, and can see that he is not being honest with the two men.
