It is vital to have strong thesis statements that draw the reader in. Knowing your texts deeply is very helpful, this will enable you to be able to refer to key quotes as well as particular events. For A-Level, examiners are looking for creative and insightful ideas which other students may not necessarily think of. To find these ideas, it is a good idea to take interesting quotes and analyse them as much as you can while still remaining relevant to the ideas of the book.
Critics are expected to be used at A-Level, but do not simply take their ideas and place them into the essay with no context. They should support and strengthen your ideas. It is often preferred that you disagree with a critics ideas as it creates a more interesting essay to read. Everything you write must be relevant to the question. This includes context also. But never let critical opinions or context distract from your own ideas. Conciseness is also expected.