One of our neighbours is Michael Gillespie, the sculptor. We went to his open studio recently, and Andrew asked him to what extent he plans in advance the way his abstract sculptures will develop. He replied that it’s like speech: do you plan exactly what you’re going to say before you start talking?
A while ago I was talking with Alan about what it’s like to do your job in a foreign language (he uses German every day and even chairs meetings in it). He said that he thinks German-speakers plan their speeches more carefully than we do. Before you launch into one of those long sentences with the verb at the end you have to know where you’re going!
Maybe it’s the same with sculpture: perhaps you would plan it more, or less, depending on the medium you were using. As with spoken languages, some materials might lend themselves to spontaneity while others would reward a more premeditated approach.