Starting by Planning or by Improvising?

This is almost less of a post and more of a question to the community: Do you tend to start a new cue/piece of music by planning it out or improvising and seeing what happens?I've heard it said before that there are two primary methods of composing. Either you envision the entire piece in your mind before picking up the pencil (or mouse, more likely), or you noodle around with material until you settle on something you like.In his book "A Composer's World", Paul Hindemith asserts the following:

"We all know the impression of a very heavy flash of lightning in the night. Within a second’s time we see a brad landscape, not only in its general outlines but with every detail. Although we could never describe each single component of the picure, we feel that not even the smallest leaf of grass escapes our attention. We experience a view, immensely comprehensible and at the same time immensely detailed, that we never could have under normal daylight conditions, and perhaps not during the night either, if our senses and nerves were not strained by the extraordinary suddenness of the event.Compositions must be conceived the same way. If we cannot, in the flash of a single moment, see a composition in its absolute entirety, with every pertinent detail in its proper place, we are not genuine creators."

Is this true for most of you out there? Do all professional composers envision a cue in a flash of lightning like this?I have had success with both methods, and I haven't really figured out which (if either) is a stronger approach.For example on a recent project I had a cue very carefully planned out. I knew where each new section would begin, how it should build, and when it would end. But when the cue was finished, it wasn't resonating with the director. For version two I tried a different approach; I let the video roll and just improvised on the piano. It required a but of polishing and finesse afterwards, but that improvisation ended up making up at least 80% of the final cue! Not to mention the fact that it only took the length of the scene to come up with, much less time than all the "planning".So to the composers out there I want to know: What do you think, and how do you work? Have you had greater success with careful preparation or carefree improvisation?Please leave your comments below and lets open up the conversation!