(Or: an Unreliable Narrator taken Beyond the Impossible.) Say, for example, you were writing a horror story. (What I'm currently brainstorming isn't this, but the problem is the same.) A group of teenager investigate a supposedly haunted house, spooky stuff happens, but eventually it turns out that it was probably all caused by some insane murderer and the ghosts merely manifested as a representation of the protagonist's personal insecurities; the ending is ambiguous on this point. How could you write a chapter or two from the point of view of a ghost, treating them as a narrator and developed character, while still questioning their existence?
So you're saying (in the example) that the ghost is a figment of the protagonist's imagination, and you want to write from the ghost's perspective? To do so convincingly, you could reveal nothing that the protagonist didn't already know, or could not actually witness/experience. You could develop their character only in a way that would make sense for the character to develop in the mind of the protagonist. I would probably suggest that first person would be the easiest way to do it convincingly - it emphasises the psychological nature of the character, rather than the physical presence (as 3rd person would)... Its a pretty complex puzzle, creating a character then creating a character inside that character's head, and then having that character narrating the action of the first character...but with enough care and planning (and ingenuity) it could be pretty clever...
It sounds like it would be hard to pull off without it sounding very cheesy, honestly. Something that would be much more poetic, I think, would be to have your main character starting to fabric memories based on his beliefs. It would start with little embellishments with what he heard/saw until his imagination caught hold and overwhelmed his sense of reality.
Choose your narrator for the best POV to tell the story from, not for a gimmick. Obviously, if te existence of the narrator is in question, it's a poor point of view to choose from, because it implicitly takes sides.
Yeah, what Dave said. If you write some portion of this story from the point of view of a narrator which may or may not exist... confusion! The fact that you have written some portion from this point of view will make concrete in the mind of the reader that said narrator does in fact exist. If you attempt to backtrack after that, I fear a very Scooby-Doo finish.
The example is a simplification, in the idea I'm brainstorming said 'ghost' appears before several others, and although having an individual, developing personality also in each appearance becomes representative of those other characters choices; in each case the ghost could literally be imagined from eg. guilt or madness, but overall becomes symbolic of certain ideology (with somewhat inevitable religious overtones) and its necessity/impact/fate in the novel's world. It's kind of hard to explain and I haven't entirely thought it through yet, the complete novel would be more reflective and exploring a few themes than a hard whodunnit plot, and the chapters from the POV of the ghost would come late enough that my real problem is showing a story and choice of the ghost itself as well as short if-only/what-if style reimagining from that ideological perspective - without confirming the literal existence of the ghost character (which would, therefore, give too much weight to one side of an ongoing consideration). Thematically, I'm sure I can justify it. Literally, I'm not sure what techniques to use, even with the mood already set, that could question the narrator's POV to a heavy enough extent without starting off with a big, cheesy 'IF'.
I love TVtropes too. All right, I think I can safely sum this one up for you. Nigel Tufnel: There's a thin line between clever and stupid, and the harder you try to get to clever the faster that line passes and the harder it is to see when it goes past. (paraphrased, of course) It's a whodunnit. Right? A whodunnit with devilish clues, a fiendishly clever villain and an incredibly skilled detective to piece them together. Over the course of the story, he finds his way past his unseen foe's deceptions and makes his way through conflicting testimony and his own inner torments to find the villain and call them out. Right? If you really need a possibly-nonexistent ghost narrator for a couple chapters in there, I'd suggest you take a long look at the story and make sure it fits. Besides, it's like a book in the second person- the first thing a reader will do is say, "Oh, that's clever." or, "Oh, this is one of those second-person thingies." Not, "Ooh, this is a cool story." There is a reason that there are so few narrator conventions, and most of them are because people notice if things aren't one or the other way. If I have to wonder whether the narrator actually exists, and thus question whether any of the book actually happens, it's going to take me away from your dashing detective and the clue list he's got. Plus, there are some themes that mesh well with whodunnits and some that don't. Guilt and innocence. Crime and punishment. Truth versus law. The legal system and its flaws. Trust and betrayal. Love and judgement. Gut feelings versus logic. Hard work versus talent. Numbers versus words. These all mesh well. Existentiality doesn't mesh too well. I'd suggest saving the possibly-nonexistent narrator concept for a book that welcomes him, like a sci-fi novel or a horror, and sticking to an amazing plot.
What I'm brainstorming isn't a whodunnit, or even slightly a whodunnit, the example was just the simplest way to describe the literary problem I'm having and apparently going to have to work out myself. Although I might take arron89's advice of switching to first person (from 3rd) since that could help indicate a significantly different perspective from the other characters as well as emphasis the character as a voice rather than a presence. But, I might risk coming across too blunt by suddenly narrating one significant point of view directly to the reader ... it would probably help to give the character something specific to speak to, a relevant trinket or something. Or technically write from 3rd person, but have the majority of the narration from the character's voice, throw in a line or two indicating 'Can you even hear me?'. That's still not quite the same as directly telling the character's own story, but if I made the trinket (/ effective sounding board) representative enough eg. bluntly, a favorite doll she made as a child, which in one way already had thoughts poured into it during a labored creation, so I can question if the character's voice then is really NEW thoughts or just memories; there's a tend towards loss in her story anyway, so you could imagine other characters wondering about old memories and if they still 'exist' in a real sense, symbolising the original question of if the ghost character literally exists or not. It would help to link the trinket and foreshadow the question with a previous story or two, and have it seen by somebody else recently, just so the mind screwy POV didn't feel AS screwy, and ideally not all arbitrary. Alright, I've lost everyone by this point, so I'll continue monologing internally.
Man. Sorry. I forgot to read the first post. I'm an idiot. Completely Missing The Point, don't you know. And that last post was completely unfollowable, but I think I got the gist of it. The character might exist but might not; they have something that ties them to this world but they aren't sure that they are still tied to the world in the same way that a physical object is. They know the object exists, and other people know the object exists, but they can't be sure that they exist the same way. They're (to use another idea) a meme, or a word, or a concept, and there's this thing that people know about that they correspond to, but they're not sure that they exist as much as they used to, and they can't be sure that what they're seeing isn't something that they already saw or something they're only now seeing since they can't tell old memories from new ones. Yeah?
Bearing in mind Cognito's very sound advice on treating POV seriously this is how I would do it. Firstly you could write the whole thing, not just a couple of chapters, as a journal or letter meant to be read by the protagonist. You could choose to keep the "real" identity of the narrator as an end-of-story revelation or you could use it to build conflict or suspense in the story (though this would mean writing in second person - always tricky IMO). If you were hellbent on writing just a couple of chapters from this "ghosts" POV then you could go down the Tyler Durden (sp?) path and just have this narrator as another character that is revealed at the last to be just a fragment of the protagonists shattered mind. Good luck.