First off, I have to express how much I love this poster (from the film's own Wikipedia page). The hand is reminiscent of that omniscient hand at the end of Lang's M, which was released a year after Murder! This time around, the hand isn't the the hand that connects the mind to the heart, but society's hand that punishes the guilty, even if wrongly.
Unlike the slew of film musicals that were popular during the advent of sound, which focused mainly on the intrigues of backstage life, Murder! incorporates theatre itself, specifically theatricality and how "art criticizes life," according to Sir John Menier (Herbert Marshall), a leading actor and playwright who, after being ridden with guilt after sitting in a jury that prosecuted and condemned innocent Diana Baring (Norah Baring) of murder, sought to prove her innocence with the help of fellow actor Ted Markham (Edward Chapman). Menier proves actress/character Baring's innocence by going back to his art and understanding the psychology of guilt that one of the jury members elucidated before being bullied into changing her verdict and justifying what gravitated him to her in the first place: her dainty look of purity, and fragility, a view justified by Menier's stature as an intellectual. (Another jury member that didn't think Baring was guilty came to the same conclusion, only that he wasn't able to justify his connection between innocence and a pretty face other than by invoking his sexual bias.) How Menier exactly makes the connection between theatre/theatricality/acting/artifice and reality/innocence/guilt is not clear to me because the scenes that elucidated the connections were far too talky. They lack Hitchcock's visual flair. Being only the third sound film of a still young Hitchcock, the awkwardness of the plainly straightforward dialog-driven scenes is forgivable, if only because the film's true core is when Hitchcock's visual style flies.
These scenes truly are remarkable. The opening scene contains much promise: a tracking shot of windows, with lights turning on and heads poking out as the camera passes to check the disturbance downstairs. The disturbance of course is a woman's scream, following the murder that sets off the film. From the beginning, Hitchcock already gives us an impression that this isn't a simple crime that involves only a murderer, the murdered, and a few witnesses. Rather, it's a murder that could only be solved if one goes back to the theatrical community that brought it about. When the police--and justice, presumably--got closest to the true killer, they are backstage, interviewing the theater company in which Baring belongs. The scene is an amazing sketch of performed reality, as the interviewed "witnesses," who presumably are being their genuine selves as they tell the police (who are themselves in costume, like th theatrical polices that they are interviewing) what they think really happened, change their dispositions as they get ready to enter the stage. The camera the switches back and forth between the interrogation scene and the play being performed (and presumably the scene that is more real to the filmic audience because it is the scene that they can see and thus prove the existence), critiques the scene that comes before, where a camera travels back and forth between two rooms (obviously studio sets) as two characters discuss the murder as they walk back and forth between the kitchen and the dining room. The performance at the theater emphasizes the performance of the two actresses as they realistically perform their roles for the film.
Just like his experiments with sound in Blackmail (1929), Hitchcock also makes use of sound well, even if they are drowned out by the middle chunk of the film where the film is all talk. The film is driven to action by a shriek. Later, when the dead body is discovered, reality is sucked back in by a loud sigh. In what is known to be one of the first example of an internal monologue, Menier's face deftly registers his guilt and his consciousness, even if his voice is threatened to be drowned out by the orchestral piece (which apparently was being recorded simultaneously from an orchestra playing in the back) that heightens the emotion he conveys with his face. Some of his filmed dialogs aren't necessarily bad either: during the jury deliberation scene, the camera smoothly travels back and forth as the power struggle between the jurors, culminating in a montage of faces as the jurors bully Menier into convicting Baring. Also, when it is Menier's turn to bully the real killer, the scene turns into one of stares and hand gestures, specifically Handel Fane (Esmy Percy) conveying his shock of being found-out with his hands as he simultaneously douses a cigarette butt.
Fane/Esmy is the pivotal character of the film because he is the very definition of performativity. A multiracial man who is also possibly gay, Fane is an actor who specializes in that very English role of a woman--really, a woman who has lost her feminine fragility which to a large extent desexualizes her--played by an effeminate man. His first "appearance" as the murderer has him running down the streets in police costume, which he later substitutes for a dress, and later again for the police costume, and finally for a dress for his final trapeze act. His final scene atop the circus tent is incredible, a portrait of a man who, in front of everybody, is finally laid bare and removed of his disguises. Acting himself finally becomes overbearing, and he enacts the hanging that was reserved for Baring. His act reveals the truth and brings the loop back to the closing scene behind the proscenium arch, where the victorious couple--Menier and Baring, with Menier finally satisfying the sexual attraction that led him to help Baring in the first place--enacts their celebratory kiss.