Last Wednesday at the restaurant in the Regency Hotel on Park Avenue, Emile Hirsch was tucked into the corner seat at a corner table, windbreaker smooshed into a ball in his lap, eating chicken noodle soup. The 22-year-old actor looked worn and pale, his fair skin all the more porcelain-like under a thick swoosh of black hair (dyed for his role in next year’s Speed Racer). He unselfconsciously plucked a cough drop from his mouth to suck down some Diet Coke. “Ow! It’s cold!” His gray-green eyes were as big as saucers.

Unlike the stars and starlets who parade through the tabloids each week, Mr. Hirsch wasn’t looking rough from too many nights in the clubs. It was work, plain and simple. He was in New York for two quick days after a stretch at the Toronto Film Festival, where he’d premiered and promoted his new film Into the Wild, and was leaving for Chicago right after lunch to tape an episode of Oprah with his director, Sean Penn. The travel and the talking were taking their toll. Cigarettes, he pointed out, weren’t helping.

Mr. Hirsch is a modest 5-foot-7 and compact, like a high-school point guard. He has glinty eyes that narrow in a flash to a teenager’s squint (he won’t hesitate to test an interviewer’s own knowledge of certain subjects), and his shoulders slouch just a little. But his boyishness extends beyond his looks. Over lunch, he demonstrated his one-eyebrow-raising, eye-crossing and tongue-curling skills (this last was borderline-pornographic); belted out a brief Sinatra imitation (“That’s why the chick is a tramp”); and enthused over magician David Blaine, who he’d met the night before. Mr. Hirsch was also impressed by the mini-grilled-cheese sandwiches that came with this reporter’s tomato soup. (“Wow! Look at that!”)

It’s this rare mix—the very adult dedication, the very youthful delight—that captured the eye of Mr. Penn, who first saw Mr. Hirsch in Lords of Dogtown, Catherine Hardwicke’s fictionalized version of Stacy Peralta’s skateboarding documentary Dogtown and Z-boys, which Mr. Penn had narrated. Mr. Hirsch played Jay Adams, an angry, intense young surfing and skating prodigy; over just about two hours, the quick-to grin Mr. Hirsch barely cracks a smile.

“He really struck me,” said Mr. Penn of Mr. Hirsch in Dogtown, via phone. (“If you hear some crunching, it’s just me getting a sugar rush out of Cracker Jacks,” he explained.) “Something in his eye, his physicality. All of it …”

When Mr. Penn first thought of making Into the Wild 10 years ago, he envisioned Leonardo DiCaprio, to whom Mr. Hirsch has been compared, in the lead role of Christopher McCandless. (“I must be like the shorter version. … I don’t know, he’s pretty tall! I wish they’d compared me to someone I could take in a fight!” joked Mr. Hirsch.) McCandless was a young, idealistic college graduate who ditched his privileged life and family to wander the West and ultimately perished at the hands of nature. Jon Krakauer wrote McCandless’s story first in an Outside magazine article and later expanded that article into the best-selling book Into the Wild.

McCandless, whom Mr. Hirsch resembles in stature, hitchhiked to Alaska in April of 1992, where he set up camp in an abandoned Fairbanks bus near Denali National Park. He managed to survive on a meager supply of rice and by foraging plants and hunting primarily small game for four months before getting sick, most likely from eating a poisonous seed pod. He died of starvation after his internal organs failed. Mr. Penn’s film doesn’t exactly celebrate McCandless—even in death, he remains a controversial figure among the adventuring crowd—but Into the Wild is a gorgeous paean to wanderlust, to the random kindness of strangers and to a landscape that, as Mr. Hirsch puts it, “doesn’t care about you.”

“Emile was a phenomenal thing to watch,” Mr. Penn said of his star. He never actually auditioned Mr. Hirsch—anyone familiar with his work in Dogtown or Nick Cassavetes’ Alpha Dog, in which he played an intense, brute drug dealer, would know he could play the role—and chose instead to meet with him periodically over a handful of months—for a quiet meal, for dinner with his family, for some good old-fashioned drinking. “My intention at the time was really to get some sense beyond whether or not he could act the part,” said Mr. Penn, “which I felt fairly quickly comfortable with.”

“I go out to drinks,” said Mr. Hirsch of one of his dates with Mr. Penn, “and meet Alejandro Iñárritu. Terrence Howard. Bono. You know. It’s crazy. We don’t really talk about the movie a whole lot. About four months go by and then he calls me [here he takes on a grave tone, imitating Mr. Penn]: ‘I finished the draft of the script and the part is yours … if you read it and like it. So come on up to SF and read it.’ So I got on a plane within a few hours. I stayed over at his place overnight and read the script, and it was just one of those fantastic moments in my life where I was really happy.”

“It was almost like time-lapse photography,” says Mr. Penn of Mr. Hirsch’s performance in Into the Wild, “because one of the demands on the actor cast to play that part was that he had to be on the cusp—I had to be able to have him go from boy to man on camera, and that was just the place Emile was in his life.”

“I can’t say enough about how … as young as he was, when we first met, and is today, this is a very talented actor,” said Mr. Penn. “But he, as a person—his desire to pursue this kind of thing at the level that he did it, in combination with being so gifted, makes it pretty exciting to see what tomorrow is going to bring with this guy.”

SITTING AT THE REGENCY, it’s difficult to see the man Mr. Hirsch portrays so heartbreakingly on film. As McCandless, he has an enthusiastic arrogance, a heady self-righteousness, not to mention a big, wild beard. In person, Mr. Hirsch’s natural intensity flickers on and off—he has a penetrating focus and a wicked smile—but he just seems so, well, young.

“This chicken noodle soup is really good!” he said, picking up his bowl and drinking the remains.

“We went to 31 different locations,” said Mr. Hirsch, cataloging his adventures as if he’s recounting a vacation. “We were in South Dakota cutting wheat on a combine; we camped out on the Grand Canyon for a week. We were doing Colorado river kayaking around, ended up doing the rapids …

“There were times when it was really, really, really hard,” he continued. “But there were times when Chris [McCandless] was on the road and it was really, really, really hard. I just knew that that was part of the commitment. I didn’t go into it thinking it was gonna be a ball. It’s amazing how you can go into it thinking it’s not really gonna be a ball, but you really don’t realize what that means until you’re doing it.”

“I think it’s important to be willing to suffer, if that’s what it takes,” said Mr. Penn of his expectations for his star.

Despite losing 40 pounds for the role (a feat necessary for the scenes toward the end of McCandless’s life), Mr. Hirsch doesn’t seem to have suffered at all, really. He shies from saying too much about his dieting (“I don’t really like to talk about the details of it because people get too wrapped up in it and it sort of trivializes it and it’s harder for people to enjoy the film,” he said), and tosses off the question like anyone could do it if they had to. And despite Mr. Penn having worked him to his physical limit, at times putting him in some danger, filming Into the Wild was nothing short of exhilarating.

“He’s a mixture of incredibly demanding, but so smart and so confident in you as a person, you don’t feel intimidated,” said Mr. Hirsch of Mr. Penn’s directorial talents. “It’s the calmest I’ve ever been on a set, ever. It’s extraordinary. You feel safe with him. And he just gives you this kind of freedom that I’ve never had before. I don’t know what my process is. I don’t have a process. But I just felt free.”

For his role as Speed in the much-anticipated Speed Racer, from The Matrix’s Wachowski brothers—expect this to be next spring’s huge movie—Mr. Hirsch basically had to do a 180. After rumoredly beating out indie darling Joseph Gordon-Levitt and little-man-on-campus Shia LeBeouf for the role, he shot the entire film, which is based on the 60’s anime cartoon, inside on a green screen in Berlin. For that film, two of his co-stars were chimpanzees named Kensey and Willy, who alternate (à la the Olsen twins in Full House) as Speed’s pet Chim-Chim. “It’s pretty amazing when you sit down for the first scene of a movie, take one, scene one, and they’re like, ‘Put Kensey in!’ and there’s a little chimp in overalls just rocking back and forth at the dinner table”—Mr. Hirsch rocked back and forth making a hoo-hoo chimp sound—“while you’re trying to read your lines.”

That was about the only taste of the great outdoors he got in Berlin. “I was really happy to be in nature so much, before being indoors for so long, that was for sure,” said Mr. Hirsch, who did manage to get some exercise by skateboarding around the set.

As we were about to say goodbye, Mr. Hirsch explained a magic trick from the night before that involves a quarter with his friend’s name written on it that Mr. Blaine magically curled inside his palm. “The quarter was bent in half,” he emphasizes. “That was like real magic.”

Did Mr. Blaine explain how he did it?

“Oh, I wouldn’t want him to,” said Mr. Hirsch. “There’s something about magic that I think is very similar to acting. You never want to reveal too much to people, because it’ll ruin it for them. You have to preserve people’s chance to watch, to enjoy it.”