What are the implications of the fact that we never meet Rusty Regan during the novel?
There are many ways to approach this question. For one, Regan, in his absence, remains the least tainted of the characters. We know he is an ex- bootlegger, but we are also told good things about him, such as the fact that he had been a good friend to General Sternwood. In short, the fact that Regan is never directly involved in the action ironically has a positive effect on his portrayal, as he never gets the chance to prove himself "bad" in the way that all of the other characters do. This irony leads us to the realization that, as Marlowe points out, it is quite possible that only in death may one be sparred from depravity the plight of the modern city.
In this regard, Regan is the only character spared from a judgment of morality. Nevertheless, while he is not involved directly in any of the action, he looms over the entire narrative, making Chandler's phantom character a clever literary device that keeps the plot driving forward. It may also be that Regan symbolizes truth in some ways, as Marlowe is search of truth and Regan is the physical object of Marlowe's search. This link between Regan and truth does imply, however, in a very pessimistic fashion, that "truth" is dead.
How does Chandler create such a dark tone? What elements and devices does he use to do so?
Chandler, who was a well-read and classically educated man, uses a plethora of literary devices to create the appropriate mood and tone for The Big Sleep. He constantly provides lush descriptions of environs, especially as regards the weather: rain drenches gun-filled nights, murders, and possible suicides; thunder almost always sounds in the background as a sign of foreshadowing.
Chandler also brings symbols into play, such as the portrait with the dark eyes at the beginning of the novel, which sets the mood right from the beginning, telling us how to read Sternwood and his family.
Finally there is the wonderful language beginning with the dark and catchy title. The Big Sleep sounds swanky, reeking of irony and foreboding. There is also the very irony of a novel with so much action being called The Big Sleep. Moreover, the language used in the novel, replete with colloquialisms and local street smarts, provides the perfect backdrop, constantly playing on the sarcasm in Marlowe's voice and adding to the dark romanticism of Chandler's.
How similar are Vivian Sternwood and Marlowe's intentions of secrecy?
Both Vivian Sternwood and Marlowe end up having to hide truths. However, Marlowe is forced to hide a truth for which he has searched, whereas Vivian has been on the defensive since the beginning—she has never sought truth, but has instead attempted to obscure it. In the end, however, the two seem to be united in the goal of protecting the Sternwood family, especially the ailing General. Although the two have differed from the outset, both the criminal and the detective, the seeker of truth, have ended on an almost equal plane. This tells us something about what the value of seeking truth, and whether such a goal is even plausible in the world of the novel.