The Stanley Parable Demo has been released on Steam, and it's been pretty awesome.
But I'll spoiler everything if I start talking about it, so I advise, press the link, get the demo, maybe try the original mod, and GET EXCITED.
And once you've finished the demo, you can answer this question:
How long did it take you to realize that the writing at the beginning was not a cutscene, but just a piece of the wall?