Writing style refers to the way which an author chooses to write. Different writing styles pay special attention to formal/informal language, sentence forms, and diction.
Given that Steinbeck's novel Of Mice and Men is a piece of fiction which takes place on a ranch during the Great Depression, the novel is written using informal language.
Informal language is one which utilizes slang, improper sentence structure (for the speakers), and hold a specific "tell" regarding where the characters are located.
The sentences are not written with worry about formal structure. Instead, the sentences vary in length, detail, and power.Some are shorter, which some could say mirror the mental ability of Lennie), while others are longer and more involved (which would mirror the mental ability of George).
In addition to the use of informal language, one can easily note Steinbeck's use of diction as matching his use of informal language. The text is filled with words ranchers would typically speak (which supports the reliability of the text for the reader).