As somebody who was introduced into the creative world through writing, there are several key differences of it in comparison to art. As somebody who merely doodled when the inspiration hit (as opposed to a well studied artist), I'll be comparing my tendencies of impulsively drawing versus impulsively writing.
Drawing is unique to me in a strange way. I feel like as you progress through your piece, you gradually get a better understanding on the direction you want to head, as well as start to picture the final product. For instance, when I doodle, it usually starts with a circle or an oval-- but I never really have an idea of how it would eventually turn out. I could be working on normal human features, before suddenly twisting into making a more grotesque zombified version. Art is special in the way that it can be extremely flexible, and is never fully finished until you happen upon a satisfactory result.
Writing, on the other hand, is extremely fixed. Rather, it is hard to compose a good storyline unless it is fixed. When people write as they go, they lose a lot of opportunities to set up the story and characters for the future, which often makes for a rather dull read. In order to combat this, authors attempt to set up new adversaries, each one more powerful than his/her predecessors. For most cases, after one or two good arcs, the story becomes repetitive and more of a chore to read and write rather than something entertaining. Because of this, most authors do extensive planning for their novels, or even whole series, before they even begin to write. This often takes up the majority of the book-making process. When the writing is ongoing, however, there is little room for change.
This is primarily my observations based on the techniques I (and most other artists) use when drawing or writing. I know there are other methods out there, but they personally seem less effective, or run into problems in the long run.