What is the theme to stand by me and how is the theme (or message) reinforced (or treated) in the film?

Watch the movie or read the book... there is so much going on in the plot you will be spoiled for choice.

I agree with redhed35, the book is actually a short story, if you have an hour or so that should be enough time to read it, and a theme should present itself easily.

By the way, the story is called "The Body", by Stephen King...