The Stranger, the first novel of French writer Albert Camus, is an example of the existentialism philosophy of the 20th century. It will help you formulate a thesis if you know something about existentialism, since Camus was one of the most famous authors who infused his writings with existentialist thinking. Very basically, existentialists (look it up) believed that we weren't born with our SELF already in us, as Sigmund Freud did, but rather that our SELF evolved from our experiences. Also, existentialists believed that each of us is not a detached observer of life but that we are open to experiences and can see every detail of life. We are immersed in life! This is one of the reasons that the main character in The Stranger describes his experiences with such excruciating, painful detail. Freedom of choice in how we act in life and how we develop in life is also very important in existentialism. We make life choices all the time!
Remember that your thesis in the paper is the one major idea, or argument, that your paper will be focused on. Once you state your thesis in the introductory (first) paragraph of your paper, the "body" of your paper (all the other paragraphs in it except for the final one, the conclusion) must discuss about three examples of how your thesis occurred in the literary work. Each body paragraph will deal with one of the supporting examples you'll discuss.
For example, let's say that your thesis, or major argument, is: "The existentialist philosophy is infused throughout Albert Camus' novel, The Stranger." Now, can you think of three examples in the book that support this thesis statement? How about one paragraph to discuss how the main character was so observant of details in his life? Then how about another paragraph to discuss how his SELF was not as clear in the beginning of the book as it was at the end, since he grew from his experiences throughout the book? Then how about a third paragraph that discusses how the character made choices in life and must now live with those choices?