In case it's not clear from her slouch or her frown or the way she rolls her eyes and sneers savagely at anything related to puppies, sunshine or apple pie, Mary Shannon is a cynic. An unrepentant, first-class, Grade-A, life's-a-big-pain-in-the-keister kind of cynic, with an attitude so roiling, hissing and steaming with flammable bile that she sets off smoke alarms.

Even if you missed all those signals, though, you'd still know that Shannon, the marvelously misanthropic federal marshal played by Mary McCormack in the USA Network series "In Plain Sight," is a dark, troubled soul. You'd know because she regularly employs an old-fashioned, low-tech method of tipping you off: She tells you so.

"In Plain Sight," like many current TV series, uses a narrative technique known as the voice-over. There's nothing new about voice-overs -- some viewers will recall series such as "The Waltons" (1972-81), "Harry O" (1974-75) and "The Wonder Years" (1988-93), in which lead characters constantly think out loud -- but the technique has come and gone over the years, like a screenwriter's version of hem lengths and tie widths.

Right now, we're in a Golden Age of Voice-Overs. Not only because many shows use them, but also because they're being used with more artistry, eloquence and flair than ever before, to set moods and tones, to deepen and sharpen characterizations, to mystify and beguile as well as to explain and elucidate. The voice-over is now a distinctive -- even crucial -- feature in many popular series.

Just as a first-person narrator can invigorate a work of literature -- think of Charles Dickens' David Copperfield, Herman Melville's Ishmael, Philip Roth's Nathan Zuckerman -- and just as the soliloquy did wonders for a certain prince named Hamlet, so, too, can a voice-over in a TV show do more than just nudge the plot along.

Admittedly, in some authorial hands, the voice-over is a lazy shortcut, an easy way to explain what's going on or to get a cheap laugh. But the writers of current series such as "In Plain Sight," "Burn Notice," "Desperate Housewives," "Grey's Anatomy" and "Dexter," and departed series that live on in motion pictures such as "Sex and the City" (1998-2004), use the voice-over as a dazzling creative device, fit for far more than mere exposition.

Without the voice-over, "In Plain Sight's" acerbic Shannon, who helps relocate witnesses in federal cases for their protection, would be hard to take.

In "Burn Notice," the voice-over by Michael Westen (Jeffrey Donovan) is informational and whimsical, functioning as a sort of textbook for spies: "Spies spend a lot of time traveling," he noted in a recent episode, as his colleagues prepare to swipe documents at a morgue in the Bahamas, "but they don't usually end up in the most desirable locations."

Not all voice-overs tell the truth; they are similar to the unreliable narrators in fiction. And not all voice-overs are created equal. "Desperate Housewives" is narrated by Mary Alice (Brenda Strong), who happens to be dead, a technique also used in the classic film "Sunset Boulevard" (1950). Carrie Bradshaw (Sarah Jessica Parker) in "Sex and the City" has the habit of turning to the camera and addressing the audience directly. In plays, this is known as "breaking the fourth wall"; on TV, it is known as "Carrie being Carrie."

"Grey's Anatomy" is usually narrated by Meredith Grey (Ellen Pompeo), but not always. The bartender at the watering hole across the street from the hospital has taken a turn: "All the interns come in here. Their jobs are complicated. So are their love lives." And how.

These days, a voice-over is much more than just a hunk of explanation attached to the start of every episode, such as "Space -- the final frontier," and the rest of Captain Kirk's greeting in the original "Star Trek" (1966-69).

We're light-years away from those clunky opening monologues. The artful voice-over, we now know, can do amazing things. The technique can humanize even a creature as reviled and inherently unsympathetic as a serial killer, as it does in the stylish, compelling "Dexter," forcing us to wonder: Can a series about a banker be far behind?