An important theme of both The Shawshank Redemption and The Green Mile is justice. Both Andy Dufresne and John Coffey are innocent of the crimes they are accused of, and yet they are both treated like murderers.

There is certainly a comparison to be made in the ways in which these innocent men are treated by a failed justice system. The contrasting elements, however, are in the fates of these two characters. While Dufresne is able to escape prison, Coffey is executed. So a good way to frame a possible thesis could be:

Both works surround an innocent man who is imprisoned and treated unjustly due to a multitude of factors that include racism, greed, bad luck, and ignorance. However, the combination of factors in the case of The Green Mile results in a deadlier conclusion than that of The Shawshank Redemption.

A thesis such as this would give you plenty of room to discuss the aforementioned themes, including the role of racism in The Green Mile, which you can compare and contrast.

This thesis prompt does limit your essay in a very specific manner: the essay needs to compare and contrast the two pieces. Both similarities and differences have to be explored. A two-part thesis statement will work well for this basic essay setup: it makes a particular point and then argues for a different point. Make the first part of the sentence a dependent clause starting with the word "although." For example, the following thesis statement could work:

Although both pieces are set in prison, the magical elements of The Green Mile allow it to explore a much wider range of themes.

That particular thesis allows you to compare the settings of each piece while also linking in shared themes. The second half of the statement then pushes the essay to explore how The Green Mile contains elements of fantasy that help it explore things of thematic importance that the other piece doesn't explore.

One way to compare and contrast the two movies is to talk about how they share similar themes and characters but The Shawshank Redemption is realistic and The Green Mile has fantasy elements. You could discuss how the fantasy elements versus the realistic elements in the story impact the narrative, the characters, and the impression of the movie overall.

Ultimately, you could say something like: Though The Shawshank Redemption and The Green Mile share similar stories, settings, and characters, the fantasy element of John Coffey's healing abilities in The Green Mile makes it a very different story than The Shawshank Redemption. Then you could write a paragraph each about three major similarities, one about the difference of the fantasy versus reality element, and a few about how the similarities and differences impact the movies.

That's just one direction to go in, of course. You could also talk about the differences between the protagonists, the differences between the jailers, and how similar settings still cause different reactions and decisions for the characters in the films.