A big question for any writer is: who is my narrator? And are they talking in the first or third person? Or, sometimes, second person (but make sure you can carry it off à la Jay McInerney in Bright Lights, Big City or don’t even bother attempting it)?
In first novels the narrator – whether in first or third person - is often the novelist, and this is understandable but it’s usually a mistake, because it means the writer ends up producing something for an audience of one: him- or herself (see Common writing mistakes #1).
The narrative voice needs to be immediately engaging and consistent throughout – although that doesn’t mean you can’t have an unreliable narrator (i.e. one who is concealing things from the reader, like Lionel Shriver’s Eva in We Need to Talk About Kevin). It just means that the unreliable narrator’s voice needs to be reliable. We need to trust that, when we open that book up after a three-day absence, we’re being told the story by the same person who was telling it to us three days ago.
A consistent, appealing voice is difficult to attain and takes practice, and yet more practice. Quite often it helps to read your work aloud – humans were originally oral storytellers, so a good story should always be read-aloudable, and difficulties with the narrative voice may be revealed by a read-aloud exercise. But once you’ve got the voice – once you can hear the narrator chattering away in your head – then you can tell your reader anything. Plot is important, characters are important, but that narrator … well, there’s no story without them.