NEW YORK — What happens to a promising young actor — a polite, 23-year-old Midwestern guy with an unusually magnetic screen presence — after he’s starred in one of the most expensive movies ever made?

“I got asked to speak at the SPAM Museum’s opening in Austin, Minn.,” says Josh Hartnett, cracking a rare smile and adding, “It was awesome. I didn’t go, because I was out of town. Otherwise, I would have.”

For Hartnett, this is all by way of saying that being famous has its ups and downs. Over dinner in an out-of-the-way alcove of the Mercer Hotel, in downtown Manhattan, he is levelheaded in assessing how his life has changed since he was cast as one of the leads in “Pearl Harbor” — and how it might keep changing, as his career continues its upward trajectory. “I’m not the head of the NRA or anything like that, so people pretty much leave me alone,” he says. “That’s where celebrity gets sticky, I think: when people start to define you as one thing or another — as something they dislike or something that they really like. I think I’m at the beginning stages of it.”

Hartnett’s ascent to stardom has already been a little bumpy. Before “Pearl Harbor”‘s release last summer, the movie, with a record-breaking $135 million-plus budget, was touted as the next “Titanic,” but it failed to make box-office history, and critics, worked up to an anticipatory lather, for the most part savaged it. Still, Hartnett’s performance did get nods, and he says that he isn’t bothered by dashed expectations for the film.

“I don’t particularly care,” he says, without an iota of attitude. “Big bummer — so what? I don’t want people to think that I’m ungrateful for what I’ve been given, but at the same time my life is not completely run by the movie business. I have friends and family that I really care about and lots of things that I love to do aside from this.” He speaks in slow, measured sentences, his deep voice an uninflected thrum, his expression mostly impassive. With a shrug, he says, “So, it’s nice when things go well, and it’s too bad when things don’t.”

At this point, things are going very well for Hartnett, who is currently appearing in “Black Hawk Down,” the Ridley Scott epic based on Mark Bowden’s book about the disastrous 1993 U.S. military intervention in Somalia. Reactions to test screenings were so positive that Columbia moved the release date up from March to late December in time for Academy Award consideration. Although the movie has an impressive ensemble cast that includes Ewan McGregor, Tom Sizemore, Ewen Bremner and Sam Shepard, Hartnett is being given star billing.

The film’s producer, Jerry Bruckheimer, who also produced “Pearl Harbor,” brought Hartnett on board for “Black Hawk Down.”

“I wanted to work with him again and took him in to meet with Ridley, and Ridley felt the same way about him,” Bruckheimer says. “Josh has done some excellent roles in the past — ‘Pearl Harbor’ being one of them — but this is a very serious movie, about a sad event in history for a lot of our American servicemen. He took on that responsibility with real dignity.”

Hartnett becomes more animated on the subject of “Black Hawk Down.” “I think it’s going to be — if this means anything — it’s probably going to be nominated,” he says enthusiastically. He is clearly proud to be in a film that addresses issues of almost painful topicality. The team of U.S. Army Rangers and Delta Force commandos who were sent into the Somali capital of Mogadishu to capture key members of a rebel faction expected a swift, surgical operation that instead became a bloody pitched battle against an angry populace. An operation meant to back up a U.N. humanitarian mission ended in the deaths of 18 Americans and more than 500 Somalis. Osama bin Laden has since claimed that Al Qaeda aided the Somali fighters. Hartnett says that working on the film, which wrapped last July, colored his perspective on the events of Sept. 11 and the war in Afghanistan, and that he hopes it will provoke a similar reaction in the American public. “This movie isn’t going to solve any of the questions, obviously — it’s a film — but it’s a really honest film about what happened over there,” he says. “And hopefully when a movie like this comes out, people will think twice about sending our troops on the ground into a land that we don’t know anything about, to be slaughtered.”

After Sept. 11, there were industry rumors that Scott was going to water down the story, make it less bleak and more patriotic. But Hartnett says he was relieved that late changes to the film only clarified the action, instead of altering what he describes as an even-handed tone. “It’s not about patriotism,” he says. “But it’s also not about — and Ridley somehow pulled this off — it’s not about being a cynical college student. You know, it’s not about being someone who sits back and examines what other people are going through and says, ‘Well, that’s worthy, and that’s not.’ It’s actually just a story, plain and simple, and that’s nice.”

To prepare for “Black Hawk Down,” Hartnett and other cast members were sent to boot camp at Fort Benning, Ga. “The boot camp for ‘Pearl Harbor’ was actually a lot harder than this boot camp,” he says. “This boot camp was informative — the other one was more like they wanted to beat the s— out of us, make us feel like what we might feel like if we were in the Army.” From there, Hartnett shipped out for four months of shooting in Morocco, where several veterans of the ill-fated outing joined the cast and crew. “That’s pretty strange any way you cut it,” he says. “We just tried to keep it as honest as possible.”

Keeping it honest seems to come naturally to Hartnett, who graduated from high school in Minneapolis only five years ago. He landed a spot in the acting program at the State University of New York in Purchase, but says he was kicked out after about six months. He headed to L.A., where he quickly landed a part on a new television detective series, “Cracker.” He calls that show’s swift cancellation “a blessing in disguise,” because it freed him to take the film roles that were suddenly being offered him, beginning with 1998’s “Halloween H2O.”

Since then, he’s completed 10 other features, from such teen flicks as “The Faculty” and “Here on Earth” to his more recent blockbusters. After completing “Black Hawk Down,” he decided to take six months off to spend time at home in Minneapolis, where he has friends, family and a girlfriend (“She’s a normal person,” he says).

Although he’s now reached a new level in the hierarchy of Hollywood stars (he was paid a reported $2.5 million for “Black Hawk Down”), Hartnett hasn’t yet altered his open-minded approach to choosing projects. “Something that I haven’t done before is usually the criterion,” he says. “And if it’s something that I have done before, and if I forgot doing it, you know? Then that’s fine, too.” He next appears as a Web designer who decides to give up sex for Lent in the Miramax comedy “40 Days and 40 Nights.” The movie is a classic romantic comedy larded with vulgar sex gags, and the strange concoction only coheres because of Hartnett’s knack for projecting authenticity. “I hope the movie comes together, because it could have been completely absurd,” he says.

Hartnett acknowledges an undercurrent of pressure from the professionals around him to do “movies that make money.” Still, he knows his mind well enough to resist taking a role he doesn’t feel right about, and for the moment he puts his trust in that same fortune that has attended his decisions ever since the day he set off from home wanting somehow to be great.

“Right now, it’s kind of chaotic,” he says. “I don’t know what I want to be. And I’m sure that’s, in a way, kind of a reflection of who I am.”