FreeGameDev

I finished Pale Echoes recently. After you complete the game you can turn on developer commentary. The game is a JRPG so the developer commentary adds a developer NPC on various screens you can talk to. Obviously it would be harder to add to an action game. Even this implementation may not be the best. I think it might be better to just start the developer commentary if you have it on and get to certain spots. If it was voice commentary instead of NPC conversation, it would allow you to listen to the commentary if you wanted or ignore and keep playing. Anyway I thought it was an interesting idea.

Dev commentary is amazing and I love it, especially in adventure games: adds replayability, give you an insight on how the game was made etc.
Even in non-adventure games (puzzles, etc.) it could work to add some backstory and getting a closer sight on gamemaking choices.