When writing a context essay, and reffering to ideas in a movie or book, should you write with the assumption that the reader is familiar with the movie or book?
I agree with #3. Assume your audience is familiar with the texts which you are using in your essay. Taking too much time to "catch up" on the plot details (in order to prove your point) will more than likely distract from your main topic and cause you to lose focus in your essay.
It will give your essay a more authoritative voice to assume your audience is familiar with your texts, and, if they are not familiar, will force them to become so in order to understand your point. Consider that most opinion writers (whether in newspapers, blogs, or online reviews) do very little in the way of summarizing their texts (whether a movie, a book, a play, etc). In this way, they narrow their audience, focus their point, and provide a true opinion.
You should assume that your reader has seen the movie, in my opinion. Don't waste so much time in your essay retelling the plot. Focus on what is specifically your topic. You want to include examples from the book or movie that support your topic.
I think that different teachers might hold different expectations here. In all questions of writing, it might be best to consult with your teacher first because they are assessing your work. In speaking with your teacher, you might be able to make the quickest assessment whether or not it is acceptable to presume that the audience knows of the work in question as you write about it. I do think that it is important to ensure that some basic background is present in the writing sample, from which deeper and more profound analysis can reveal itself. I do not think that a plot summary and unnecessary detailing of plot elements are needed at the cost of the thesis statement in the writing sample. It might be important to state some basic elements of the movie or book that will enable you to go ahead and set out what you wish to prove in the course of the paper. Yet, some teachers will have different expectations and it is here where it is important to gain clarification. In the end, if the work sample is out to prove something about the movie or book, few would want to wade through unneeded information. It is here where I think it's important to ensure that what is included is relevant to the proving of the thesis statement, something that is present in all analytic writing.