'Writers are liars my dear, surely you know that by now?'
— Neil Gaiman in Sandman: Dream Country
I don't know if there's a genre for stories written within stories, but if it was its own genre, I'd be all over it. The other example that comes to mind is 'Murder Mysteries' by Neil Gaiman, a short story that retells the fall of Lucifer. Heck, I remember loving a Lion King fanfic that covered Scar's revolt from the Pridelands, and a NiGHTS: Into Dreams story that explained Reala's statement 'There is no NiGHTS!'*
I've had my own hand at it in the past, writing fanfic and world building fannish stories that cover gaps between episodes and scenes in Starsky & Hutch. Somewhere I've a huge book full of plot points and character development for the TF2 classes. It became my own personal challenge as Valve added their own canon to the game to tie it all together, and somehow make it all work in context. I even explained Max's Severed Head, despite my own personal grievances with the sillier hats in TF2.
Still, I'd love to read more, and there must be more out there. It can't only be at the hands of fanfic writers. I think there's a real skill in being able to somehow manage it successfully (not to say that fanfic writers are in any way unskilled). To be able to weave a story between something pre-established, to stay true to that source material, and yet craft it from another angle, takes so much discipline and commitment. It's setting a very strict set of instructions you can't disobey, and finding a way of skirting around them in the most creative ways possible.
Which leads me to wonder, and ask; are there any other examples of that? Stories that a the tale behind the scenes, or that somehow approach things from another angle? I'm open to any form of media going - television, movies, comics, whatever you can throw at me. I just want to see the other side of the coin.
* Something tells me someone pedantic will inform me that Reala doesn't say that, and that everybody was originally misinformed due to a shoddy translation. I don't care. That line sent shivers down my spine when I first played the game and it chills me now to recall it. So there. Go away, fact bringers. Shoo.