Yes, if you have the time, you'll definitely want to read the entire novel before you analyze an extract from that novel.
To me, there are at least two main steps when it comes to analyzing an extract from a novel:
1. Read the extract closely, as you would a short story, and pay close attention to the language, ideas, and patterns in the extract.
2. Develop an understanding of how the extract relates to the novel as a whole. Does this selected passage focus on characterization or foreshadowing or a particular theme or one of the high points in the plot, for example?
This strategy is often a very good way to write a paper about a novel. Rather than try to talk about five or more extracts, focus on just one extract, analyzing it closely and making clear statements about how this extract relates to the novel as a whole.