I found the script book fascinating, particularly because some of the major changes from the poem were made because of questions Avery wrote down when studying the poem in High School. ("Who was Grendel's father?" for instance.)

The script imposed plot, which the poem does not have. It has "story" which is not quite the same thing. It also imposed a classic tragic structure (which I suspect was Gaiman's contribution.)

(Oh cripes, I've volunteered for the book to film adaption panel at Orbital, and this is gonna come up, I can feel it right now. I admire this script, and I am not going to be popular.)