There are a couple of other points that come to mind with the comparison of Darkness and Crucible. One is that, while we now tend to think of the late '40s and early '50s as the height of Cold War hysteria, anti-Communist movies and plays didn't usually catch on with audiences in this era; anti-McCarthy plays and movies (The Crucible, High Noon) tended to do quite a bit better. The Cold War just didn't produce a flood of popular Cold War culture in the way that WWII brought on a bunch of successful anti-Nazi movies; and despite the stereotype of the era and its culture, the anti-anti-Communist messages tended to be more popular than the Cold War messages. The other point is that if you're going to write a political play, you'd be best advised to make it about your own country, or at least find some way to tie the play's message in with an issue facing your own country. I'm not fond of The Crucible, but it is an American political play about American political issues. Darkness at Noon, taken from a Russian source, is primarily about Russian issues and makes very little attempt to apply its themes to American politics. It's not hard to see why American audiences wondered what this had to do with them.
Kingsley's adaptation is generally very faithful to the book, with much of the original dialogue and all the same basic story points and flashbacks. He doesn't try to add any fake uplift to the ending, either. The comparison with The Crucible is interesting here: Proctor in The Crucible refuses to make a false confession and save himself, instead dying a martyr for the cause of truth, justice, and the Arthur Miller way. Rubashov in both the novel and the play of Darkness at Noon confesses to crimes he didn't commit in a Stalinist show trial -- after he has been informed that he will be executed no matter what he says or does. There's no martyrdom here, and no uplifting moment; Rubashov confesses because it's his last opportunity to do something for the Revolution to which he's dedicated his life -- even though he knows that the Revolution has gone wrong, and finally comes to suspect that there was something wrong with it at its core.
There are a few changes. Arlova, Rubashov's secretary who loved him and was sentenced to death when he wouldn't come forward to defend her (Gletkin: "To save your neck." Rubashov: "To be able to go on working." Gletkin: "Without a neck one cannot work. Hence, to save your neck?") is here renamed "Luba Loshenko," based on the immutable principle that all Russian women are named "Sonya" or "Luba." Her part is expanded, too, with more flashbacks to her relationship with Rubashov and a big scene of her interrogation just before her death. This provides a respite from the otherwise almost all-male cast, and Most of the other changes are just to drive home points that are made obliquely in the book; near the end, various bits from the book are combined and adapted to give Rubashov a big speech about where, when, and how the Revolution went wrong, and the play ends with a new dialogue scene between Rubashov and Gletkin, the epitome of the brutish, unimaginative "Neanderthal" that the Revolution has created:
GLETKIN: Enemy of the People Nicolai Semonovitch Rubashov, before you are executed, have you any last wish?
RUBASHOV: One. If I could only make you understand where in the very beginning we failed.
GLETKIN: These are your last words. Don't waste them.
RUBASHOV: You don't build a Paradise out of concrete. My son...
GLETKIN (quickly, distastefully): I am not your son.
RUBASHOV (after a long pause, sadly): Yes, you are. That's the horror.
(When I first read this bit, I thought Gletkin actually did turn out to be Rubashov's son, which would have been a hell of a plot twist for the third-act curtain.)
Politically, the play is an exemplar of old-fashioned liberal anti-Communism, one of the few such examples in movies or plays of the time (most of the anti-Communist movies of the era were of the right-wing variety, like Leo McCarey's bomb My Son John). This is driven home in a number of scenes, most clearly in the scene between Rubashov and Richard, the stuttering writer whom the Party turned over to the Nazis for writing the wrong kind of anti-Nazi propaganda pamphlets. In the book, Rubashov denounces Richard's pamphlets for admitting that the Party is in trouble; you and I can make mistakes, he explains, but the Party is infallible. In the play, Rubashov has a somewhat different objection to Richard's work:
RUBASHOV: We sense a certain sympathy with the Liberals and the Social Democrats.
RICHARD: The Storm Troopers are... Sl... sl... slaughtering them too, like animals in the street.
RUBASHOV: Let them! How does that affect us? In that respect the Nazis are clearing the way for us by wiping out this trash and saving us the trouble.
RUBASHOV: The Liberals are our most treacherous enemies. Historically, they have always betrayed us.
That's probably as close as Kingsley gets to unveiling a relevant message for American audiences, namely that Communists are the enemies of liberals. It might have been relevant, say, ten years earlier; by 1950 it was kind of old hat.
Kingsley, whose best and best-known plays were Dead End (with the Dead End Kids) and Detective Story, was not a subtle writer, nor a particularly deep writer. But at his best, he was one of the best writers of Broadway social-problem dramas, through his ability to create strong characters and his willingness to allow more than one point of view in his plays; Detective Story has memorable characters throughout its large cast, and an especially strong protagonist who is portrayed as wrong-headed but not villainous. A play like Darkness at Noon, where the problems portrayed are less ambiguous in nature than those in a crime drama like Detective Story, and where the characters are not of a kind that Kingsley had encountered personally (which didn't allow for the slice-of-life realism of Dead End or Detective Story), may not have played to his strengths. A strength that does remain is Kingsley's superb sense of stagecraft, his eye for the strong theatrical gesture. In the book, the prisoners communicate with each other by tapping on the walls in code. This device is retained in the play (the characters speak the words they're tapping out, so we can know what they're saying), and the tapping sound is used for dramatic effect in several important spots, most notably at the end: as Rubashov is led away to be shot, one prisoner starts drumming on the wall, followed by other prisoners, followed by the tapping/drumming noise spreading throughout the whole prison, building to a crescendo, and then fading away: a perfect sonic counterpoint to Rubashov's offstage death.
The role of Rubashov was played on Broadway by Claude Rains, who was presumably great (when wasn't he?). Kim Hunter, Stella of A Streetcar Named Desire, played Luba.