For the past few years, Jake Gyllenhaal has won acclaim playing geeky but sexy young brooders in cult movies like Donnie Darko and The Good Girl. So it was something of an adjustment to act—or try to act—in a special-effects disaster flick like The Day After Tomorrow, with a tidal wave flooding New York and twisters shredding Hollywood. "They would shoot a scene of me, and I was like 'independing' it," he says, referring to the loose "indie film" approach to moviemaking. "They were like, 'There are 800 extras behind you, dude, you gotta turn [this way for the camera].'" The 23-year-old finally saw the light when Dennis Quaid, who plays his father, had a little talk with him. "I remember Dennis sitting me down one day and just saying, 'You gotta chill out. It's an action movie.'"

No mistaking that: The Day After Tomorrow is a $125 million fantasy about a world gone cuckoo because of global warming. Along with an $86 million opening-weekend box office and mostly awful reviews—although The New York Times noted approvingly that Gyllenhaal "has a way of infusing even his most desperate lines with a hint of knowing sarcasm"—the movie has generated controversy because of its dire warnings about global warming. Gyllenhaal totally backs the movie's message: "My generation was brought up on Earth Day." Also in the film's favor, he notes with that same knowing sarcasm, "Hollywood gets destroyed."

And Gyllenhaal (pronounced JILLenhall) gets a career boost. The movie, in which he plays a 17-year-old stuck in a snowed-under Manhattan, has transformed him into a box office hunk just a few weeks before his girlfriend Kirsten Dunst, 22, returns to the screen in Spider-Man 2 with Tobey Maguire. Gyllenhaal, in fact, was briefly attached to Maguire's role in Spider-Man 2, and is now considered the front-runner to fill Christopher Reeve's blue tights in the upcoming Superman movie.

He certainly is looking the part. "I exercise almost every day," he says. "I go to the gym every day, I run, I play sports, but not on the weekends." When he and Dunst were in Manhattan working on their newest films, they were spotted lifting weights and walking the treadmill side by side at a downtown gym. Says a trainer: "They were very loving and so cute."

They've been that way since September 2002, when they were introduced by Gyllenhaal's sister, actress Maggie Gyllenhaal, 26 (Dunst's costar in Mona Lisa Smile). "Jake has dated a lot of high-profile girls," including Natalie Portman, says one friend, but "Kirsten seems like one of the most down-to-earth." Says Dunst: "I am really happy and in love." The couple, who share her $1.7 million Los Angeles home, wear gold promise bands (she keeps hers on a chain around her neck) and seem unbothered by the paparazzi who trail them when they walk Atticus, their German shepherd mix, or go out for coffee. "Kirsten and I know what's real and what's not," he says. "We know what happens when you are both actors."

Gyllenhaal—who grew up in Los Angeles and spent summers on Martha's Vineyard—has Hollywood in his genes. His father is director Stephen Gyllenhaal, 54, and his mother, Naomi Foner, 58, is a screenwriter. Godmother? Jamie Lee Curtis. Family friends include Paul Newman, who gave teenage Jake driving lessons on the racetrack. Gyllenhaal's big break came when he was 18, starring as a budding rocket scientist in the 1999 drama October Sky. Despite his new action-hero status, the actor remains partial to smaller, gentler movies that will keep showing his range. Upcoming projects include Proof, a drama with Gwyneth Paltrow; a re-release of the mind-tripping fantasy Donnie Darko (2001), now with extra footage; and Brokeback Mountain, costarring Heath Ledger, based on the Annie Proulx short story about a romance between two cowboys. "It's a story about two people who are drawn to each other, who aren't gay but who fall in love," Gyllenhaal says.

Meanwhile, the actor—long admired by movie critics—has developed a newfound respect for action heroes. Making Day After, he says, "I started worshipping Will Smith."

Tom Gliatto. Sean Daly in Los Angeles and Amy Longsdorf in New york City

  • Contributors:
  • Sean Daly,
  • Amy Longsdorf.