When looking to rhetorically analyze the movie Knives Out, one would need to look at both the textual and contextual pieces of the of the film. The textual part (that is to say, the substance) can be analyzed by taking into account the relationships between characters in the movie. Many of the characters are portrayed as greedy, mainly the people in the family. Their interactions, words, and individual motivations must be looked at.
As for the context, it would be interesting to analyze the filmmakers lives, where they were, and beliefs about inheritance and money.