This is a movie about clockwork. I'm not just talking about the fact that publishing magnate Earl Janoth, played with admirable creepiness by Charles Laughton, is obsessed with the precision of clocks - both in and of themselves, and as a metaphor for the ceaselessly mechanical way he runs his business. And I'm not talking about the fact that every other line uttered in the first thirty minutes of the film is something like, "do you have the time?" or, "what kind of time do you call this?" And I'm not even talking about the fact that the two most pivotal moments in the movie - the murder (this is, after all, a film noir - of course there's a murder) and the movie's climax - both completely revolve around clocks.
What I mean when I say that the movie is "about clockwork" is that the enjoyment one derives from The Big Clock comes from the fact that the film is perfectly timed. It's like a good Agatha Christie story, in that everything that happens, happens exactly when it needs to. Every action of every character leads inevitably to the other actions of the other characters. Nothing happens arbitrarily. Nothing happens accidentally. I don't mean that characters don't have accidents, or do things that they hadn't planned. But every accident any character makes is set in motion by something a previous character has done. Much like the gears in a clock, the characters in this film have preset paths they are forced to follow by the fact that the other characters are forced to follow their preset paths by the fact that the other characters are forced to follow their preset paths... and all this seeming chaos works together to create one unified effect - the never-ending ticking of the plot.
Mirroring the actions of the obsessive Earl Janoth, director John Farrow never allows The Big Clock to stop ticking. Because as Janoth suspects at the beginning of the film and knows for certain by the end, when the clock stops ticking, chaos reigns. And this chaos might be good for Janoth's tired and belittled workers, but it is bad, bad, bad for the guy at the top. Fortunately, Farrow never looses his cool, never breaks the clock, never sends his world into chaos. The movie just keeps moving inevitably along, unbroken and entertaining.
Also, this movie has what may be the most thematically perfect murder weapon in the history of cinema. I mean, the film's The Big Clock, it's obsessed with clocks, so yeah, it makes sense that the murder weapon is a clock. But it isn't just any clock. It's a sundial. A mechanical clock is perfect as long as it keeps on ticking, but a sundial has all sorts of potential for error. A human has to line it up exactly right for it to work, and humans make mistakes. Earl Janoth (who is the murderer - not really a spoiler as the film never tries to keep it a secret) absolutely despises the idea of human error. To him, the sundial represents everything that was wrong with the pre-industrial world: it wasn't totally controlable. Also, a sundial has to be used outside, and in Janoth's world, the act of going outside is an act of rebellion. The sundial, to Janoth, is like a sick, twisted, abomination version of the one thing he loves - his clocks. And it is the mere presence of the sundial in his mistress' apartment that sets him into a fit of rage. Moreso even than the fact that she was cheating on him.