Say what you want about Marlowe, but this tough guy tells it like it is. And he tells us about the other characters straight up. He's always observing the folks around him and commenting on their appearances or behavior.
Take Marlowe's first visit with General Sternwood, for example. He describes the General's physical appearance very explicitly: "His thin clawlike hands were folded loosely on the rug, purple-nailed. A few locks of dry white hair clung to his scalp, like wild flowers fighting for life on a bare rock" (2.2). We can clearly picture General Sternwood's thin, frail body. But we also learn more subtle pieces of information beyond the General physical appearance. In Marlowe's interesting simile, the image of wild flowers fighting for life is also an appropriate metaphor for the General's own struggle for life.
Since Marlowe is a detective, he has a keen sense of observation, and when he gives us details, there's usually a reason behind it. So as you read, pay close attention to his descriptions. There's usually a lot more going on than first meets the eye.
Chandler spent a lot of time obsessing over the speech and dialogue of his characters. He wanted his characters to sound real, to talk the way normal people talk in everyday life. That's why you see so much slang and vernacular speech peppered throughout the dialogue in The Big Sleep.
Let's take a look at one of the conversations between Marlowe and Carol Lundgren:
"Who are you?" [Carol] snarled.
"Friend of Geiger's."
"Get away from me, you son of a bitch."
"This is a small gun, kid. I'll give it to you in the navel and it will take three months to get you well enough to walk. But you'll get well. So you can walk to the nice new gas chamber up in Quentin."
He said: "Go — yourself." (16.89)
The catchy dialogue here is a classic example of the lively, fast-paced, tough street talk of Chandler's hardboiled style. We learn a lot about the characters—namely that they're not exactly upstanding citizens—from the way they talk and express themselves. Language and speech become a way of asserting power in the gritty streets of L.A., where tough talk is a crucial means of survival.