Lesson One in Screenwriting 101 is Show Don't Tell. In a well-made film, the story advances by action; in a lesser film, the story advances by exposition, the characters describe the action; in an inferior film, the story advances by an unseen narrator.
The latter technique is drawn from books where, because of the absence of visual images, the unseen author must describe the images and the action. In books, the art of telling the story is in the author's word craft; in films, the art of telling the story is in the director's camera and editing choices.
Three current films provide three different directorial approaches to cinema storytelling: Gus Van Sant's Milk, David Fincher's The Curious Case of Benjamin Button, and Woody Allen's Vicky Cristina Barcelona. The first two use a principal character as an on-screen narrator to advance the story. In Milk, the narrative is carried by Sean Penn as the title character dictating his story into a tape recorder; in Button, the narrative is carried by Cate Blanchett's character as an old woman, revisiting her life through her scrapbook. Both films are, by any standard, excellent productions. Each is a strong candidate for the Academy Awards, with powerful performances, rich production values, and important themes, but both are repeatedly interrupted by returning to the narrator telling the story. Each film has enough going on in the action to propel the story forward without having to resort to this narrative device.
In Woody Allen's film, the narrator is an unseen male voice that comments on the story rather than telling it. Mick LaSalle, the San Francisco Chronicle movie critic, put it best in his rave review, "Voice-over narration gets a bad rap because it's often added as an afterthought to films that don't hang together in the editing. But in "Vicky Cristina Barcelona," the narration was built into the design, and it's used extensively and effectively, placing us securely in the story."
Show don't tell.