What does that mean specifically regarding the comparison essay?
Very simple: the subjects must be easy comparable, so you don’t need to work too hard to point out their similarities or differences.
Yes, the topic is assigned, yet how it will be presented depends on you.
For example, in the framework of drawing analogy between two books of your choice you are free to select books: Depending on the selected type of comparison, you will build the paper in a particular but consistent way.
Nevertheless, there are tested strategies that turn drafting and writing process into a tolerable task, if not total fun.
You will begin with selecting the type of compare/contrast essay that appeals to you.
Writing a compare and contrast essay may look like ‘mission impossible’ because the process of comparison can take hours and even days of hard work.
So it is naturally a scary job to undertake a task that has no visible limits.
Just remember that while looking for traits to match you should correlate the same parameters of explored objects, like plot and motifs of books or movies, price, quality and branding of some products or color, taste and juiciness of different sorts of oranges.
Comparing color of one sort of oranges to scent or price of other sort is meaningless and thus cannot be considered true comparison.