I don't know what to say about this film that hasn't already been said, so the following is just some thoughts. One this is clear to me; this is one of the most influential noirs ever made. I know because I've seen its influence without even knowing it. For one example, I saw Roman Polanski's Chinatown about a year ago for the first time, but I really should of seen this first. That film, and many other postmodern noirs contain infinite homages to this film.
Philip Marlowe (Humphrey Bogart), while quick-witted and sharp tongued, knows no more than the audience who sits behind him, following his progress through the lamp-lit back rooms, casinos and mansions. It is the undertaking the journey, and not comprehending the twists and turns of the plot that is really important in the film (but if you're curious, Wikipedia has a thorough summary... which still managed to confuse me). This film is famous for its fabulous writing, which combines the screen-writing skills of William Faulkner and Leigh Brackett, who is actually a woman, despite the gender ambiguity of her name. According to imdb, Howard Hawks liked her because she wrote "like a man." What exactly does that mean? Without sentimentality? I'm not sure either.
Anyway. Howard Hawks' film represents a familiar story-line: the detective goes off to work on a seemingly simple case, but things are darker, and more convoluted than they initially appear to be. Marlowe starts out looking for one thing, and ends up looking for something completely different, and of his own volition; he is paid off by Vivian (Lauren Bacall) to stop pursuing the case. But like all good hard-boiled detectives, he is not influenced by money, but by his personal sense of right. There is no sense of dread, of imminent fate, as there is in many classical noirs (like Double Indemnity). The viewer becomes enmeshed in this entropic environment, with no sense of whether the right people will be punished, or who deserves punishing. In the end, Marlowe and Vivian get out of the mess they're in, but that's it. They are left looking at each other, a little bit awed by what they have just seen--unsure of what to make of the new order of the world.
One noticeable theme in this particular film, as in many older films, is the gender construction. As played by Bogart, Marlowe is the epitome of masculinity, while Lauren Bacall's Vivian serves as a perfect foil for him because of her atypical femininity. She is sultry and powerful at the same time. At the outset, Bogart confronts Sternwood (Charles Waldron), the tired, wilting old General surrounded by a bed of Orchids, as if awaiting his own funeral. He is past his prime and emasculated, which is represented by his inability to drink or smoke (oh, and the wheelchair). By comparison, Marlowe, with the perennial cigarette dangling from his lips, is wiry and virile. We get the feeling that, in a way, Marlowe is looking at a mirror of his future self.
As the film goes on, Marlowe's quick wits are not enough to save him from the entropic elements of the universe. In the scene depicted above, he is tied up, literally and metaphorically, while Vivian hovers over him, lighting his cigarette for him. She then saves him by using her own cunning, a position usually reserved for the male. In this film, the women are on an even playing ground--dangerous because of their ability to beguile, lie and evade with the best of their male counterparts, while in the process, turning them into empty and powerless figures like General Sternwood.