What should I base my ideas for set and costume design in a production on?

When given the task of describing and justifying design choices, the very first thing a student should do is establish whether their production will be in the style of naturalism, or if they intend to introduce non-naturalistic elements. This choice itself ought to be based on the nature of the production, and if a non-naturalistic approach is taken, it ought to be justified. A naturalistic approach to lighting, costume and set ought to take into account factors such as context, setting, location, period, time, character psychology and relationships, occupations, and wealth. This is all with the goal of creating the most true to life aesthetic possible, though the design of the set in particular ought to keep theatrical practices in mind, so as to prevent any hindrance to the audience's ability to see and hear the production.
Non-naturalistic design ought to keep this last point in mind, but should focus on finding a grounding cornerstone, a particular message, atmosphere, or aesthetic on which to base design decisions. This central idea should be based solidly in the text of the play, and given a thorough justification. This justification established, there is a relative freedom to explore design options.
Importantly for either type of design, each and every design decision included in an essay, for sound, lighting, set, and costume, must be described clearly, given a basis in the text, and justified, particularly in regards to the effect it is intended to create in the minds of the audience.