Excellent question. The description in the movie doesn't make any sense: if she can see the green land and cannot turn, it is very difficult to imagine the position of the "utterly dark abyss." The reason it is so confusing is that the movie combined two things that were separate in the books. Both lines are popular, so the screenwriters included both of them, without caring that they didn't make any sense when they were put together. In the book, Eowen describes her current state as "I stand upon some dreadful brink, and it is utterly dark in the abyss before my feet, but whether there is any light behind me I cannot tell. For I cannot turn yet. I wait for some stroke of doom." Meanwhile, the "green land" refers to Faramir, not Eowen. Faramir dreamed of "a great dark wave climbing over the green land and above the hills". Separately, both visions make sense. Smashed together, they're just confusing. This is why books are always better than movies. :) !

Pretty much. When I read it, I pictured the speaker standing in a cliff, looking at the green valley below, as a huge, dark wave crashed over it, hiding all the green in total darkness. Does that make sense?