There was a terrific interview with Katherine Paterson, the author of Bridge to Terabithia, on the Disney Insider right before the movie went into theaters. It is based on her son's experiences and he was the one who really pushed for it to be made into a movie.
For those who have read the book and only seen the trailers, there is not much fantasy in the movie, maybe 10 minutes tops. It does keep very true to the book though some things are left out of the movie, but nothing major.