We, as in my 4 person indie group, have an entire whiteboard that looks like graffiti for prototyping our game ideas. We look at it every day and choose which idea draws us in the most when we near time to start a new project. We also randomly draw characters that seem interesting even if they have no gameplay associated with them. This works but doesn't always seem to give us the best understanding of our ideas. We tried a rule that if you come up with a game idea, build a very basic prototype and show us, but that also wastes time.
So, my question is "what is a good way to prototype game ideas". We need a balance between getting the idea across and not skipping it because we don't understand it, but also not wasting time building up a ton of ideas that will never be touched. What is an efficient way to describe the major aspects like gameplay, art feel, special features, audience, etc.