For the sake of discussion, in my own work I've broken this down to two things.
Self-reflexivity is when a story seems to, time and time again, refer back to itself, or to other stories, or to use the ideas of storytelling or performance a lot. For example, both Batman Begins and Casino Royale used performance, the construction of a persona, as justifications for Batman and James Bond's characters, respectively. Comics, by this definition, have been self-reflexive since the first moment someone said, "Say... how about we have the star of your comic book come and do a guest appearance in my comic book?" That's the original intertextual gesture, or intercomical if you like.
Metatextuality in comics, "metacomics," is when the fourth wall is broken, either explicitly or implicitly (though the latter is harder to pull off), and when that breakage is left in pieces on the floor, instead of being fixed either after the fact or at the end of the narrative. As I said before, Promethea and Sandman don't let reality be. It's constantly mixed with fiction in ways that just don't add up, and that's the point of metatextuality and metacomics. It's what they do, and it has deep implications in terms of both the power of media (mediation between the thing and the thing that represents the thing), and how much "reality" is always mediated (by icons, language, cultural assumption, even the functioning of our own sensory apparatus, to steal N. Katherine Hayles' phrasing).
The interesting thing is that, in comics, genuinely metacomical moments are almost always followed by self-reflexive "fixes." The first time Barry Allen travelled to Earth-Prime was a metacomical moment, but then Earth-Prime became just another element of the extended DC multiverse, and it reverted to an intercomical element. Earth-Prime became just another setting for superheroes. The What If? and Imaginary/Elseworlds stories were extra-diagetic narratives; they existed outside of the main story but used elements of it. However, the moment those stories were made to exist within the main narrative (i.e., their "universes" actually exist within the DC or Marvel multiverses), they revert back to being intercomical. For example, the Tangent characters, originally conceived as a way to fill in skip weeks back in the 90s, have just shown up in Ion: Guardian of the Universe as refugees of Infinite Crisis.
The point here is not to set up some kind of hierarchy in which the metacomical is inherently superior or more sophisticated than the intercomical/self-reflexive, but to show that comics have been sliding from the purely self-reflexive to the more and more metacomical since the late 60s (depending on how you recon it), and if that slide has finished (which I don't think it has), then we live in a time that has experienced the breach of fiction into reality, and we're trying to pick up the pieces, but this time in a way that betrays that we realize that the pieces will never actually fit back together, and indeed never did fit.
Posted by orion at February 1, 2007 7:39 PM