I generally agree that a good quotation citing gives complete information. With movies, books, etc., it just doesn't happen all that often.
From
http://www.classicsondvd.com/harvey.htm
Harvey is a rabbit with a pedigree. Mary Chase's enduring play had already won the Pulitzer Prize for drama and had enjoyed wildly successful runs on Broadway and in London before it was brought to the screen. The screen version thankfully stays very close to the play, primarily due to the fact that it was adapted by Chase herself, along with Oscar Brodney. The pair was able to open the play up a bit to make it cinematic, but kept the expansion within the context of the play. In doing so they realized the best of both worlds.