One of the most exciting aspects of writing is when fascinating new elements seem to emerge from the hidden depths of the imagination. This process happens all throughout the initial writing, but I find that the most interesting elements tend to come out during revisions.
Sitting down to re-work a manuscript is sometimes daunting. Knowing that things need to be added, and that certain complications need to be solved can often lead to static thinking. One tries to logically solve these problems, but logic usually leads to boring solutions. However, somewhere along the line, a strange image or thought will pass through your mind, setting free the little demons that hide there. Once struck, the creative energies gather these snippets of madness, piecing them together in impossible ways, and suddenly those formerly difficult problems are given brilliant conclusions.
It's important for any artist to be comfortable letting their demons run the show every now and again.