Before prototyping, comes pretotyping.
Pretotyping is a way to test an idea by building an extremely simplified, mocked or virtual version of that idea. Gives fast feedback on product validation.
Say you wanted to pretotype a new mobile phone, you could build it out of wood or cardboard quickly to validate the physical dimensions of the phone.
Pretotype a mobile app - Sketch each screen quickly on an index card before even touching HTML or Photoshop. Navigate through the pretend screens.
This is a fail fast technique. If the pretotype is validated then you can move on to further investment with a prototype.