(What follows is a lonnnng (and I mean "long") discussion of a tool for storytelling, plus the movies Fallen and Election, the book and film versions of The Shawshank Redemption, and an unanswered question about the series finale of Quantum Leap...)
Screenwriting books tell you: NEVER USE NARRATION. Narrating your story is telling the story instead of showing it, and that is the sin what is cardinal. Except when it isn't.
It's easy to notice if narration is over-explaining something. There's a decent mid-'90s film called Last of the Dogmen which suffers from a narration track (spoken by Wilford Brimley) that tells you about (it seems) all of the background: this character reacts that way to that character because of this event that happened before the film began; that sort of thing. The subliminal message I got was "the studio executives were terrified that the film wouldn't make sense without this narrated info," which explains the (to me) spoken overreaction. Again, decent film as I remember (and it has an early David Arnold film score), but hobbled.
An even more blatant example of the same problem is the Denzel Washington horror film Fallen; it's narrated by Washington, as a detective who witnesses a bad guy's execution (unleashing a malevolent spirit which had been imprisoned inside the killer and which then wreaks havoc through other people), but A) the info he adds this way is often unmotivated by, and unconnected to, the onscreen action, making you wonder if there's a hidden reason he keeps speaking to you, and B) at the end, when Washington's character battles that dangerous spirit one-on-one -- and dies -- we learn that the voice we've been hearing throughout the film is not Denzel's character, but the malevolent spirit...except it doesn't ever take over Denzel's character, it just battles him. It's a cheat. The two characters (yes, I have to refer to a floating spirit cloud of evil as a character, for lack of a better term) have nothing to do with each other, other than battling each other, but if it had been the voice of Some Guy instead of the great voice of Denzel, the audience would at least unconsciously be wondering "Who's speaking?" I'm almost positive that the narration was added at the last minute; I had heard at the time of Fallen's release that it was significantly edited right before coming out, and narration is one of the easiest things to change in a film. But does that narration have a purpose? That's harder.
Before that, Frank Darabont struggled with whether to use narration in The Shawshank Redemption. The original Stephen King short novel he was adapting, Rita Hayworth and Shawshank Redemption, is all first-person narration by "Red," telling us about several decades of prison life and the influence of Andy DuFresne...a technique which King used so that this aw-shucks man telling us about this amazing guy Andy is really, inadvertently, revealing himself to us. It's a way of getting into the mind of a character who is a convicted killer, but is still decent, someone who truly is paying his debt to society. We know he's a decent man, who owns up to the worst thing he ever did ("only guilty man in Shawshank," Red says of himself), because we spend enough time in his mind to know he's no Manson or Dahmer. Anyway, Darabont had written films before, I don't think using narration in his earlier work, and he knew from other writers the accepted wisdom that narration was The Lazy Way To Tell Your Story, but as he took a whack at his Shawshank script he realized he couldn't get rid of the narration; it was too much a part of the story's basic structure. He was stuck with it. But it works, both in the book and the film, because it's very careful about perspective. At a key late moment, when we're looking at Red in the cemetary next to the prison, we clearly can sense that's he's thinking of Andy and the changes in Red's life because of him. Then the film fades to black...and then Red has his last session with the parole board. In the book, there's a line across the page separating two chunks of text; the film's fade-out serves the same purpose, demarking a change in time to when Red is being released (or has just been released, in the novel). After that, Red's telling of his present and his future, which he had rarely spoken of earlier. His life changes, so his perspective changes, so the use of the narration changes. And it's very smoothly done.
A more fun, and extreme, example is how the movie Election (another book-to-film adaptation, by the way) shifts the narration from character to character -- each a little deluded in her or his own way, or at least with personal perspective so skewed that the entire film skews in response: the movie's tone, even its music, changes whether it's Matthew Broderick, Reese Witherspoon, Chris Klein, or Jessica Campbell narrating. What makes Campbell's character feel betrayed and overdramatic (and is scored with rumbling, disturbed music) at the same time makes Klein's not-too-bright character think "Cool. Things are cool," and the music's bouncy and happy then, while Witherspoon (in all her Tracey Flick aggressively-smiling glory) is acting like she's above it all but really isn't, and we can tell that from how the music is around her. It's not smooth, and it's not meant to be smooth, the way it is in the deliberatly paced Shawshank; it's instead showing us that the world of the story is an unsettled place where things are going to blow up in the characters' faces, for the purposes of comedy and satire.
And sometimes...that perspective provided by the narration gets forgotten, or messed up, or made unclear. I was a fan of the TV series Quantum Leap, and from the first episode, the main character, the time-jumping Sam Beckett, narrated what was happening. (There was also the opening description, of the female voice saying that Sam was "setting right what once went wrong" while "always hoping that his next leap would be the leap home...") About halfway through the show's run, an accident causes Sam to switch places with project observer Al, so that Al's stuck in the 1940s while Sam briefly returns to both the late 1990s (still in the future at that time in the show's run) and the secret lab where he'd started the Quantum Leap project. And that episode was narrated, too. In the narration, Sam clearly was speaking to us from the perspective of some time after, sometime following his leaping adventures. His memory, "swiss-cheesed" by the original time-jump, is fully restored; he remembers that his memories were also fully restored when he reached the '90s again. But, in bringing Al out of the past and back to the 1990s, Sam gets sent time-jumping again, with his memory once again with holes...and the show went on for a few more seasons.
In Quantum Leap's final episode, Sam sees the work of another "leaper" like him, as well as another character who may be God or, at least, someone or something deeply powerful. This other character knows who is leaping, and, more importantly, why: these people, not just Sam, are "fixing" things that need to be fixed. And he and Sam have a heart-to-heart, and Sam decides to do an unauthorized leap that truly needs to be done (revealing to Al's wife, who'd been told that he'd died, that Al was alive and safe), and he leaps out of that...
...and where does he go? Words appear on screen: "Sam Beckett never returned home." OK, but he went somewhere. For five seasons, he'd been narrating from that somewhere/somewhen/whatever. We're left hanging. Is Sam on some other plane of existence? Is he safe? Is he alive? Dead? Something not quite either? Had that been worked out in any way earlier in the show's run? It's an unanswered question.
If I ever meet Quantum Leap creator Donald P. Bellisaro, that's the question I'd ask him. That and, "Hey, could you set me up with the actress who plays Abby on NCIS?".