Some creators achieved great success working like that, such as Fernando Meirelles, Larry David, and Woody Allen.

I'm much more of a planner myself - like Spielberg and Hitchcock. Not that I'm even close to them, of course. But I admire their method. It is not uncommon for most things to occur precisely as planned, sometimes to the second. But you gotta be prepared to think on your feet. I think that's the message behind those sayings.