Correction: An earlier version of this story incorrectly identified Edward Norton as the star of the 2003 film “Hulk.”

There’s a moment in “Marvel’s The Avengers” — in which some of the comic-book world’s most classic figures come together in a cinematic explosion supercollider — when Iron Man (Robert Downey Jr.) and Thor (Chris Hemsworth) are fighting each other in one of several alpha-dog fights. Eventually, the strait-laced Captain America (Chris Evans) joins the scuffle, which, when it finally ends, leaves them looking at each other with dazed looks of “Now what?”

That’s the very question that has bedevilled Hollywood for the past three decades, as movie studios have sought ever-newer ways to mine­­ comic books, Saturday matinee serials and outdated TV shows for superheroes who lend themselves so perfectly to the special effects, allegorical fantasy and wish fulfillment that cinema is uniquely suited to deliver.

Christopher Reeve ushered in the Superhero Generation in 1978, when he first donned the cape and big red “S” in “Superman.” With self-effacing leading man Michael Keaton, Tim Burton infused new artistic vision and tonal gravitas with “Batman” in 1989. Since then, studios have been clamoring to stake out their patch of the lucrative superhero turf, raiding the vaults of Marvel Enterprises and DC Comics for anyone with a cool costume and ready-made mythology to build a franchise around.

The result is that, for the past decade, movie audiences have been fed a steady diet of comic-book movies. Like the NBA, which makes sure basketball fans always have a game to watch, Hollywood has made sure that there’s almost always a spandex-encased vigilante swinging, flying, kicking or Hulking out on a screen. An escalating arms race in visual effects and post-production dingbats has proceeded apace, leading some naysayers to complain that, with the computer-generated imagery, motion-capture animation and spectacle over substance that comic-book movies entail, acting will be all but irrelevant.

It turns out they were wrong, as “The Avengers” gratifyingly proves. The biggest — and surely most delightful — surprise of the movie is Mark Ruffalo, a sad-eyed, soulful actor whose bona fides include lauded performances in such respected art-house films as “You Can Count on Me,” “The Kids Are All Right” (for which he was nominated for an Oscar) and “Margaret.” In “The Avengers,” Ruffalo plays Bruce Banner, a scientific genius who, because of an early mishap with some errant gamma radiation, turns into the Hulk when he gets angry.

On paper, the Hulk doesn’t immediately look like the kind of material an actor of Ruffalo’s sensitivity and intelligence would be drawn to. In fact, many of Ruffalo’s fans — with visions of Nicolas Cage’s career dancing in their heads — first greeted the Hulk casting news with trepidation bordering on outrage (not Our Mark!).

But in “The Avengers,” Ruffalo infuses Banner and his big, green alter ego with all the emotional truth and low-key, amusing nuance that have made his more “serious” performances so compelling. He’s still Our Mark — watchful, understated, vulnerable, oozing boyish sex appeal — just on a bigger canvas.

And, yes, greener.

As a relative newcomer to the “Avengers” team, Ruffalo jousts most entertainingly with Downey, who in 2008’s “Iron Man” infused a refreshing jolt of subversive self-mockery into a genre that was teetering on the brink of fatal self-seriousness. His glib, cock-of-the-walk portrayal of shallow billionaire-genius-playboy-superhero Tony Stark strutted in sharp contrast to Christian Bale’s laconic Bruce Wayne in “The Dark Knight” that year. In fact, it was Heath Ledger’s unhinged performance as the Joker that made Chris Nolan’s stylish but ludicrously solemn film so memorable.

Downey, Bale and Ledger ushered in a new generation of gifted actors who have migrated from indie to the IMAX. Michael Fassbender, a festival-circuit favorite in films such as “Hunger,” “Fish Tank” and “Shame,” nailed his portrayal of Magneto in “X-Men: First Class” last year; Jeremy Renner — who delivered such a galvanizing turn as an elite bomb squad specialist in “The Hurt Locker” — proves just as handy with this year’s weapon of choice, a bow and arrow, in “The Avengers.” In a few months, Andrew Garfield — who burst on the scene five years ago in a tiny, tautly made psychological thriller called “Boy A” — will pull up the tights and pull down the mask in the reboot “The Amazing Spider-Man.”

If Garfield can do for Spider-Man what Downey and Ruffalo have done for Iron Man and the Hulk, he’ll add proof that it’s the old-school art of screen acting — not CGI, IMAX, 3-D or any other abbreviation — that can lend lift and heft to an otherwise generic product.

In a form that’s always on the verge of exhaustion — sagging under recycled plots, repeated motifs and empty fetish — it’s the humans, not whiz-bang effects or lavish set pieces, who resuscitate it with verve, warmth and invention. And for a profession vexed by technological creep and salary squeeze, it took comic-book movies — not quiet chamber pieces or bravura character studies — to prove that actors still matter.

Of course, even the most accomplished performance can be for naught in the hands of the wrong filmmaker: Even a performer as fleet, appealing and charismatic as Hugh Jackman couldn’t transcend the alternately tepid and bombastic “X-Men Origins: Wolverine.” Nor could Eric Bana overcome director Ang Lee’s fatal pretentiousness and over-reliance on effects in 2003’s misbegotten “Hulk.” (Edward Norton took his own classy, sober-minded turn at the role in 2008 with “The Incredible Hulk,” which brought scale and respectability back to the property.)

But now that Ruffalo has so capably stepped into those torn trousers, it’s difficult to imagine anyone else in the role. Reportedly, he’s signed on for six more “Hulk” movies, both with and without his “Avengers” teammates. “Now what?” might be a cinematic question for the ages, but the answer will always be: First, find the right actor.