Why does Steinbeck begin "Of Mice and Men" with a detailed description of the countryside before the characters enter?
John Steinbeck is setting the scene for George and Lennie before they enter. Actually, each chapter begins with Steinbeck's use of sensory imagery. This brings the reader into the scene and allows him/her to experience that setting as the characters might. Also, in chapters 1 and 2, Steinbeck uses 19 animal references. These act as symbols that both characterize George and Lennie as well as foreshadow later events in the plot. For example, a water snake slithers by in chapter one. The snake symbolizes transmutation (change) because a snake sheds its skin and leaves the old one behind. This could foreshadow changes in the relationship between Lennie and George. The snake reappears in Chapter 6, but is eaten by a water heron. The change for George and Lennie is disastrous, and nothing will ever be the same for these characters. If you make a list of all the animal references and then use a dictionary of animal symbolism, you can make connections to the characters and the plot.
Steinbeck loved the area he writes about in his books: the Salinas Valley, Monterey, the areas adjacent. In the opening passage of Of Mice and Men, he describes an almost Eden-like place (consider the title of another of his works, East of Eden), filled with water, light, beautiful trees, and animals. It's peaceful and natural, and is disturbed only by the humans who invade it.
George, and especially Lenny, do not mean to pollute this small paradise. Lenny is depicted as what the Elizabethans would have called a "natural," someone who is innocent and childlike. But in his case, his immense size and strength, which he cannot completely control, betray him and the small animals he loves.
Steinbeck is especially adept at using lyric passages, such as the one that opens this work, to deepen the human tragedies he depicts—and Of Mice and Men is truly a tragedy.