Like a flower
Marlow meets with the General for the first time inside the greenhouse. The general is extremely sick and the greenhouse is the only place where he feels safe and is able to breathe properly. When Marlow arrives, the General explains to him that just like an orchid, he can only live in a warm climate. The fact that the General compares himself to a fragile flower that needs a certain environment to live highlights just how dire his health situation is and that he needs to be taken care of constantly in order to survive.
Knight and Marlowe
From the beginning of the novel, Marlowe identifies himself with the figure of the knight. From the moment Marlowe sees the stained glass portraying the knights saving the damsel in distress, he begins to compete at a subconscious level with the said knight. The two knights are compared and the reason behind it is to highlight how more competent Marlowe is compared with the knight on the stained glass.
Mars the horsman
When Marlowe meets with Mars, he notices that Mars is dangerous and describes him as having a hardness commonly found in horsemen. By comparing Mars with a horseman and by taking into consideration the way Mars is described, he is quickly established as the villain of the novel and as a rigid character that is unable to change himself.
Homosexuality and weakness
Homosexuality is used in the novel as a metaphor for weakness. In the 1930s being a homosexual was considered as being a taboo and the idea that man had to be manly and take matters into their own hands was promoted. Men who were homosexuals were not seen as being strong people and their homosexuality was seen as a weakness. The reason behind this is that homosexual men were slightly more feminine and manifested a bigger interest in womanly subjects. Because of this, the men who are homosexuals in the novel often act irrationally, behave in erratic ways and are more emotional than the rest.
The big sleep
The "big sleep" is the name given to the novel and is also the central metaphor in the novel. Since the novel is a detective novel, many characters end up dead or badly injured. Thus, the big sleep refers to death and is a metaphor for death used throughout the novel.
