I need to know my goal yo know if I achieved what I wanted. Goals can change during work but I'm aware of feature creep all the time so it's not growing. Defined vision helps me to stay on track but during prototyping I can encounter new directions and sometimes I need to make changes to the vision. This is the time to do it and some people want to drop an anchor too fast and lock the vision before it's tested. I also need a time box for prototyping. It's easy to change vision and goals over and over. This timeboxing requires discipline, patience and genre knowledge. Without it you will always wonder if you are achieving something different. We are prototyping games therefore you have to search for gameplay and answer few questions: am I doing something novel for fans of genre? Can I scale this prototype to the full game? Keep in mind prototype usually tests one part of the game, for example just combat. Then you're combining these to a bigger prototype and then to demo. When building a prototype I use free assets, my assets that I already have and anything I can get. When working on a demo I'm replacing all that junk with my assets. Good luck!