I am Jonathan Blow, game development person; ask me stuff.
I am a multiclassed game designer/programmer and I run a small game studio in San Francisco. A couple of weeks ago we released a game called The Witness; for Witness-related questions, we'll have some other members of the team on here answering questions as well.
Questions are not limited to Witness-related stuff, obviously!
If you do ask about Witness stuff, please spoiler-tag anything spoilery; don't presume that most people here have played or finished the game.
Edit: The following other members of the team will be answering Witness questions:
Andrew Smith, programming: anchsm
Luis Antonio, modeling and texturing (they hate it when I say that and they want me to say "Artist"): castorpt
Orsi Spanyol, also "Artist" and designed some of the puzzles: Orsi_Spanyol
So, this is a pretty good variety of people who can answer questions about all portions of the game. So you can feel free to ask detailed stuff about the engine, the modeling, et cetera!