3. Production
This is the phase in which most people are brought on to accomplish a task together as it may require additional artists, programmers, sound designers, or any other role that the studio needs.
For a game, the concept is given form through a 2D or 3D representation. Every important tool or mechanic behind the concept to make it work in a demo is built. A key factor in a game demo is if it is fun or not. Even if it sounded fun, this is the key proving ground for it.