Rambly, speculative, Sunday night blog post.
Every interactive story is haunted by the ghosts of those choices not taken: and also, by every choice previously taken. Here's an academic sort of take on this and here's an enthusiastic one.
This often means you've replayed the story, but it can also mean you've spoken to someone else who's played through, read an anecdote or a wiki, or seen a content glitch give you extra information.
This produces some powerful effects, because you can know what might have happened in the halo of alternate playthroughs - but this is a semi-accidental product of treating it like a real story.
So what kind of effects could we base deliberately on this kind of multiple, penumbral experience? A few sketches.
Information that only makes sense in aggregate: This is a very transmedia sort of thing. We see the Red Constable paint his lodgings in pigs' blood, but we don't find out why unless we take a different route through his story - one that omits the pigs' blood ending. There's no way to fnd out what's 'really' going on without sharing informatin.
Repetition as a sort of tone poem. A deliberate effect created through repetition: a mood or effect that doesn't work until you've seen the Stolen River rise three times and enjoyed the cumulative effect.
Feedback story-tailoring. The simplest example of this is the visual novel with endings that are locked off until you've unlocked other endings.
Metagrandfather Clock feedback. If characters make a point of different lines each time you go through, to show you different facets or alternatively a distinct story, that could be fun. Or unsettling. Oddly enough, GTA IV does something like this, with the repeated mission briefings.
Something I've always liked. In Somoza's The Athenian Murders, there's an (entirely fictional) device called eidesis, which relies on embedding a message through events in the story that make no sense to, and are ignored by, the story's characters: events the reader can nevertheless assemble a secondary story from. (Ice-Pick Lodge's notorious Pathologic does something similar.) I don't know exactly how this would work. References to a red glow in the street, and a stench of burning, in three dozen unrelated choices. It becomes apparent eventually that there's an erupting volcano presented through eidesis, invisible except to multiple play-throughs. This is a special case of information that only makes sense in aggregate. I keep thinking there must be something else cooler to do with it.
Our endgame strategy. One of the things that got me thinking in these directions. I can't say too much about that yet, of course...
Other thoughts? Which ghosts should walk?