Sure he's got the great voice, the musical smarts and daring, the cute grin, the word-nerd appeal to fellow crossword-puzzle freaks.

But David Cook had a stealth weapon, as he rose above the pack to be a co-finalist for this year's "American Idol" crown.No, it's not that strange scarf Cook often wears trailing from his back pocket like a talisman.

His special mojo: theater training.

The 25-year-old Cook may be known as a hard rocker. But he took enough acting classes, and co-starred in enough student shows, to also qualify as a true Theater Geek.

His teachers from Blues Springs, Mo., have spilled the beans on how much the adolescent David loved to belt out those Broadway show tunes.

For evidence, get thee to YouTube, where you can watch clips of the teenage Cookie playing Riff in a high-school staging of "West Side Story" and taking pratfalls ("Make 'em laugh, make 'em laugh ... ") in a student version of "Singin' in the Rain."

Cook's drama chops gave him the chutzpah to croon that schmaltzy, soaring reverie, "The Music of the Night," from Andrew Lloyd Webber's "The Phantom of the Opera" - and emerge unscathed.

Thespian versatility also gave Cook - and, to a lesser extent, third-place contestant and college drama major Syesha Mercado - a boost in other key ways.

So listen and learn, wannabe rock gods and "Idol" hopefuls. Here are some tips Cook may have picked up from his own personal "High School Musical" saga:

Tell the story: Cook's best performances on "American Idol" have been when he's chosen story-songs, and made them two-minute musical playlets.

Actors learn how to hook people fast with a compelling narrative. And Cook has used his facial expressions, dramatic arrangements and agile whisper-to-roar voice range to affectingly put across tales of a single teen mom ("Billie Jean"), a lonely Englishwoman ("Eleanor Rigby"), a guy waking up to life ("I'm Alive").

Play the part: Getting into character is, apparently, a foreign concept to even some of the most vocally gifted 2008 "Idol" contestants (such as Carly Smithson and Cook's co-finalist, David Archuleta). They don't seem to grasp that singing is, basically, acting via music.

Cook gets it. He played the swaggering, sexy dude with his covers of Free's "All Right Now" and the Beatles' "Day Tripper," then softened up as the wounded yet gracious ex-lover (for Mariah Carey's "Always Be My Baby") and the swoony swain (Lionel Richie's "Hello").

As "Idol" judge Randy Jackson once told Cook, with his usual eloquence, "Dude, I really kinda believed you right there, y'know what I'm saying?" Yes, Randy, we kinda believed him, too.

Look the part: Cook understands that, on camera, every outfit is a costume.

To sing "Day Tripper," shrug on that leather jacket. For Dolly Parton's bluegrass ballad "Little Sparrow," wear that humble, white, Sunday-go-to-meetin' shirt. And to stir up fan-girl frenzy, do your hip casual/formal thing: dark shirt, jacket or vest, tie worn over the collar.

And Cook's changing array of hair styles (head and facial)? Like another "Idol" wardrobe chameleon, Seattle's Sanjaya Malakar, Cookie kept 'em guessing by changing up his tresses - from scraggly emo locks and soul patch, to fashionably choppy designer 'do and scruff-beard.

Practice, chill, practice: For the actor to dissolve into the role, you shouldn't see the fluster.

While rivals like Brooke White and Jason Castro betrayed their nerves and inexperience by screwing up lyrics and choking on raw fear, Cook worked the stage (and camera) with assurance - the kind you get by rehearsing hard and performing since grade school. (And, yes, by playing a gazillion dive-bar gigs in Kansas City and Tulsa.)

Save the theatrics for the performance: Acting teachers often remind students to "leave it on the stage." And except for the odd method-acting genius (say, Oscar winner Daniel Day-Lewis), that's sage advice.

Thanks to "Idol" media frenzy, most Cook fans know: 1) He has high blood pressure, which spiked one night and sent him to the hospital after an on-air "Idol" performance; and 2) his older brother Adam is seriously ill with brain cancer, yet managed to travel to L.A. to cheer him on.

That old "show must go on" mantra may sound corny, but it's the credo actors live by. Since all is fair in love and talent contests, Cook could have milked his family situation for sympathy votes. He didn't, saving his passion for his songs.

Sure, he's only human, and got a little weepy when his bro was in the "Idol" live audience. But he shed those %*+*! after finishing his song and taking a bow. Now that's a trouper