I recently worked on an adaption of a short story into a short film. It is very hard working with the original author. Sometime I would think of some "brilliant" dialog or backstory to add, but the author looked at it reminding me it changed everything the story meant, and not necessarily for the better.
When I write original scripts I'm working on story and the best way to tell it at the same time. With adaptations you're simply trying to find the best way to tell the story which limits you to a degree, but is very rewarding when you get it right and have done the original authors work justice.