It's a decision that most authors seem to make on whim or instinct - who's telling the story? The character who communicates the story to us, whether first or third person, present or omniscient, reliable or unreliable, is integral to the flavour and feeling of the story itself. Imagine Jane Eyre in first person, or Robinson Crusoe with omniscience. If Nick had been someone we could trust to tell us about Gatsby.

Having multiple characters convey your story can add huge depth to it - if that's what you want - each giving their own layer of interpretation and bias. I wrote an honours thesis about using multiple unreliable narrators as a form of characterisation, as Jonathon Safran Foer does in Everything is Illuminated. Multiple narrators allow you not only to explore the arcs of more than one person, or take the story beyond the limitations of a single vision, they can add a rich layer of complexity to the existing story.

And therein lies the problem. In speculative fiction, extra narrators often seem to be added whenever convenient, to convey information that the author feels the reader should know that the current narrative character doesn't or can't. But introducing multiple narrators comes with a price - and a high one.

When a reader starts reading with a character, they're prepared to connect and invest emotionally in that character. They'll be interested in trying to work out the character's drives, they'll bother to remember the character's name, and the names of those important to him or her. They care about the character.

Introduce another character, and the reader's interest drops. They already put a lot of effort into bonding with that first character, and now you want them to make that effort all over again with a totally new one. If you don't make a compelling case here, your reader won't bother. They'll either skim ahead to where the first character returns or they'll put the book down altogether.

Each time you introduce a character, the same thing happens - you make the reader start all over again. Worse, you weaken the connection they have with previous characters. Once you get to the fifth or so character, you reader start to wonder how many characters there'll be, and whether there's any point connecting to them - is the reader even going to see these people again? They will no longer care about any of them, or bother to remember their names. If you've also made some of them disposable - they're going to die, or have horrible things happen to them, the reader's empathy for the others will be shot. Why bother being interested in their fate when they're a dime a dozen?

Think of it as a fade-to-black in a movie. Sometimes they're effective, and if they're positioned appropriately and used properly, they add greatly to the movie. But a movie that seems to only give you five minutes of film and then fade away, and then another few minutes and fade away will have you restless in your seat. The fade-outs are cutting your immersion in the movie, distracting you.

Multiple narrators, especially in genres like spec fic, where there's already a lot of information for the reader to process, must justify their existence. Two narrators is generally acceptable, three can work if you're careful. But if you find your cast climbing beyond that, take a look at whether they're all truly necessary - are they all serving the themes and needs of the book? Or are you perhaps trying to fit two books in one? Are some of them just being messenger-boys delivering plot or world-knowledge? If so, you can almost always find a way to integrate the information into the current character's narration, even if they don't actually realise said information. Readers are smarter than you think - they don't need everything spoon fed.

The exception to this is perhaps multi-book sagas, where the story passes between characters across a number of books. If that's your aim, just remember that each character needs a good length of time to establish themselves with the reader, and to re-establish the connection after you switch, especially when the reader hasn't seen them in a while, like at the start of the book. Chopping back and forth between a dozen characters in the first hundred or so pages will leave your reader utterly confused and largely uninterested.

That's not to say it's impossible to write a story composed entirely of characters who stick around for three pages and are never heard from again. But it's hard to do successfully. I mean really hard. At the "if you think you can do it, you probably can't" level of hard. If you want multiple narrators in your book, make sure they're pulling their weight.