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past three decades, as movie studios have sought ever-newer ways to mine­­ comic books, Saturday matinee serials and outdated TV shows for superheroes who lend themselves so perfectly to the special effects, allegorical fantasy and wish fulfillment that cinema is uniquely suited to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