AFAIK whilst some creatives do fully conceptualise the work in their heads then set out to externally reproduce it, the more normal approach is to have a less complete notion and then create something via the process. IE, a musician might have a melody in their head but then they will play it on the instrument and experiment with different variations and accompaniments to see what sounds good and build on it that way, rather than sit and think of an entire piece based on that, then play it out loud.