I just read Ed the Happy Clown and the edition that I had contained a running commentary on its creation. Cartoonist Chester Brown mentions that the comic was mostly improvised and that he used the structure of writing/drawing a panel a day without a full script to rely on.
The result is an exciting and out-there story with constant twists and turn. As the story progressed and neared some sort of conclusion, Brown does mention steering the story in a certain direction and have a plan of sorts to lead towards, but for the most part he uses the improv technique.