Halfway through the trailer for Ben Stiller's remake of The Secret Life of Walter Mitty, the title daydreamer—stuck in the rat race with boring men in boring suits at Life magazine— lifts his eyes from his desk toward a photograph on a bulletin board. Now here's the payoff, when staid reality breaks into excitement, and the nebbishy lead is drawn into the world of fantasy and adventure that Fox is betting will win our $12 and two hours come Christmas. It's a photo of Sean Penn.

In the picture-in-the-picture, Penn stands under a storm-swept sky, mountains in the background. He's got a vintage camera in his left hand and beckons, movie-magically, with his right for the viewer to enter. Surrounding him is a small cluster or village of what appear to be ramshackle tents, lean-tos made of corrugated tin and tarps or skins. The movie isn't even finished yet, so I can't guess what exotic hotspot this is supposed to be, or what he is supposed to be doing there (rumor says arctic exploring, photographing, maybe both). But we all know, instantly, who he is. The real-life jostled, matted hair and Van Dyke, the wrinkled safari shirt, and the tent camp are as much a part of his image as the rest: It's Sean Penn playing Sean Penn.

By now, Sean Penn the actor has fully merged with Sean Penn the activist/aid worker, recombining into a globetrotting rendition of the most interesting man in the world. Image and resume are always inseparable in Hollywood, and most of us have been living in greater Hollywood all our lives. Actors pal with world leaders, and the leaders cameo in the actors' projects in turn.

For all Penn's warzone parachuting and activism in the past, the critical juncture in forming his new image seems to have come during the months he spent living in Haiti after the January 2010 earthquake there. It's no real surprise that the adventurer Penn image would pop up in a Ben Stiller production; the comedian preceded Penn in the Haiti philanthropy thing and could be spotted now and then in the real-world post-earthquake camp that our media and cultural shorthand came around to calling "Sean Penn's."

Penn first set foot in Port-au-Prince one week and two days after the earthquake hit, heading a team of medical and aid workers he'd assembled with the funding by one of his wealthy neighbors in Malibu. They called themselves the Jenkins-Penn Haitian Relief Organization, or J/P HRO. I was living and reporting in Haiti when the earthquake struck, and within a few weeks I'd heard rumors Spicoli was in town. As the correspondent for a news service as celebrity-obsessed as our audience, it was part of my job to keep track of such things. But I didn't pay much mind. John Travolta had landed and left already with his team of Scientologists, and Wyclef Jean was running arond with his own NGO. Another famous face with yet another five random letters and a slash in the overflowing alphabet soup of humanitarian, military, and contracting organizations seemed like puff-piece news for a quieter time. But I was wrong.

In part it was because, unlike Travolta or often even the Haitian-born Wyclef, Penn stuck around for weeks at a time after the quake. In part it was because Penn came in with so much money and clout, in a country where being wealthy, connected, and white makes you instantly more powerful than basically everyone who actually lives there: Penn became a force in Haiti. After just a few weeks of on-the-ground training in his first real-life aid worker role, Penn was officially recognized by the International Organization for Migration with a senior role as camp manager for the displacement camp near where his group had almost accidentally set up shop. In essence, without a vote or speaking five words of Haitian Creole, Sean Penn became the mayor of a town of more than 40,000. It would be easy to go too far.

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This is what we know about the biggest public health scare in the nine months after the earthquake: On the morning of May 3, 2010, a fifteen-year-old boy named Oriel showed up at the NGO clinic above the Pétionville Club golf course with a scratchy throat and fever. Noting a gray hue at the back of his throat, the doctors made a startling diagnosis: diphtheria, a dreaded airborne bacterium that attacks the lungs and throat. Few of the foreign doctors would have ever seen the disease: Thanks to widespread use of the diphtheria-pertussis-tetanus vaccine, it has all but been eradicated in the developed world. But the aid workers knew that in Haiti, where just half the population had received a full three-dose course of the vaccine, it remained endemic. There had been 605 diagnosed cases in Haiti over the previous five years.

The boy's symptoms had been going on for six days. He needed a hospital, fast. At any time, the infection could close his throat, suffocating him to death. The doctors' first choice, the Medishare tent hospital, had caught fire after being struck by lightning a few hours before. So they went to the General Hospital. But the NGO on duty there, International Medical Corps (IMC), was reluctant to introduce a highly contagious infection into a damaged facility housing patients with tuberculosis and AIDS.

That's when the director of the NGO that ran the clinic in Pétionville stepped in. By now, Sean Penn had become a major figure in the reconstruction. He was a constant presence at cluster meetings and Pétionville bars, working as hard as anyone in the zone. Even veterans who had dismissed him as an arriviste now praised the Jenkins-Penn Haitian Relief Organization, whose experienced aid workers, given a Haiti-only focus and independence of USAID strictures, had shown agility in dealing with the problems of the quake zone. On a more personal level, it also turned out the skills that made leading men were also useful in the humanitarian world. Amid the bureaucratic blustering of the cluster meetings, a confident speaker who could command the attention of a rotating cast of strangers, and the media, would win the most consistent platform for his ideas.

Oriel may have reminded the actor of his own teenage son, who months before had been hospitalized after a serious skateboarding accident. When the General Hospital seemed to close its doors, the actor loaded the boy into a pickup truck and stormed off in search of another facility—perhaps not realizing that there wasn't one with the equipment needed to save the boy if he stopped breathing.

But the NGOs at the General Hospital hadn't categorically refused. Those hesitant to take the patient were overruled. Others argued that so long as Oriel was isolated, the risk would be manageable. It seems the communication breakdown got worse from there. The standard treatment would be to administer a dose of diphtheria antitoxin (DAT), which in Haiti was housed only at a warehouse operated by the health ministry and World Health Organization. Doctors at the hospital said that Oriel had to be admitted before the warehouse would release the antitoxin. Penn would say, two years later, that he had to ensure he could get the antitoxin before Oriel would be allowed in the hospital. After a confusing couple of hours, Oriel finally arrived at the General Hospital's green gates—reportedly just after 5 p.m. The warehouse had just closed.

Penn played his advantage. With a few phone calls, by his account, he soon had the American Red Cross, WHO, USAID, U.S. Centers for Disease Control and Prevention (CDC), Haitian officials, and the command of the U.S. military's Joint Task Force- Haiti scrambling to procure medication for a fifteen-year-old boy.

It seemed that Penn could marshal more resources than even the president, much less any other camp manager, could imagine. Some responders appeared eager to please a movie star. Others may have feared that he could break them: Penn was “like his own walking accountability mechanism,” said Timothy Schwartz, an anthropologist of Haiti who befriended the actor after the earthquake. “Everybody shapes up . . . and if you don't do it, he's going to scream at you and denounce you to the world. And it works.”

Even in the United States, DAT takes a while to procure: The CDC, which usually quarantines it at major airports, promises to deliver it only “within hours.” In Haiti, despite infinitely more difficult circumstances, Penn, and a CDC representative were able to reopen the warehouse and get the serum. Furious over the hours of delay, Penn lashed out at the hospital and then went back to camp. “We had a hard night, we drank some rum, and we went to sleep,” he recalled.

But Oriel's story had a heartbreaking end. As his days-old condition deteriorated, the doctors put him on a respirator. At some point overnight, perhaps during a shift change between IMC and the overnight doctors from Partners in Health, Oriel was left alone. When doctors returned, the breathing tube was dangling, and Oriel was in a coma. The boy had probably awoken, panicked, and ripped the life-sustaining tube out. The doctors revived him, but the lack of oxygen to his brain likely dealt an irreversible blow. Two days later, he was dead.

But unlike most others, this case would be remembered. The next night, an enraged Sean Penn appeared on Anderson Cooper 360 in an olive-drab collared shirt, his unwashed hair slicked back. Voice shaking as he jabbed his finger into his palm, the actor-turned-activist-turned- aid worker unleashed a righteous fury at aid workers inured to tragedy and a Haitian healthcare system that, for all its improvements since the quake, was still horribly inadequate. Then he issued a warning: “It's just the very beginning,” the actor nearly shouted at Cooper. “This is a disaster, and a bigger one than the earthquake, waiting to happen.” Unless the NGOs were pushed to “get off their butts,” Penn railed, “people are going to die en masse.”

CNN ran the interview with the title “Epidemic Threatens Haiti.”

The interview set off alarms. Aid workers scrambled to ensure there was enough DAT on hand for the few dozen cases expected in Haiti each year. (There was.) Haitian medical authorities ordered still more. Aid groups planned an education campaign for camp managers and communities about the disease. I called Anshu Banerjee, the leader of the health cluster. He told me Oriel's death was a tragic but isolated case. The CDC had said since the quake that diphtheria cases were possible but a large-scale outbreak unlikely. Moreover, UNICEF had an ongoing campaign to vaccinate 888,000 people in the camps against diphtheria and other diseases. But in large part because of the attention Penn drummed up, the health cluster continued to focus on diphtheria for weeks.

Penn's outrage was understandable. Had the NGOs coordinated or had there been a healthcare system Oriel's parents could have turned to days before, the boy might have lived. But no diphtheria epidemic broke out, nor did medical experts expect one. The megaphone that enabled Penn to procure extraordinary help for the boy also made his unscientific pronouncements more reckless, adding another dose of panic to an already panic-driven response.

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In the years since, the actor's power has only grown. He would go on to become a key ally and confidant of Haiti's U.S.-backed pop star-turned-president and an internationally demanded expert on Haitian affairs. J/P HRO, which hired some of the best talent in the aid world, has become lauded as one of the most effective aid groups in the zone. But at the same time it has become one of the most influential nongovernmental organizations in a country that critics also rightly point out is run by them. Penn has even joined the ranks of other celebrities in taking on a political title of his own, credentialed as an ambassador of the republic. To a large extent, many who work in Haiti have said, that's because of the access and often unfocused power Penn showed in that early misstep: He can go onscreen, denounce your group, and threaten your precious tide of donations and funds. Few want to risk crossing him. And that goes for us in the media too: As the philosopher Jay Newman has written, “celebrity and authority have a way of enhancing one another, especially in the eyes of a journalist.”

The image that has resulted, the one Stiller played off in the new 'Walter Mitty,' is a mix of Hollywood and Haiti. A regularly repeated legend has Penn trading his Malibu home for a muddy tent “surrounded by 40,000 desperate people,” protected mainly by a Glock 9 in the actor's waistband.

It was never strictly true. In Haiti, Penn and his associates lived in a large barn-like structure protected from the elements, inside the well-guarded perimeter of a U.S. Army forward operating base on the grounds of a country club, up a steep hill from the camp he managed. (The Army withdrew from Haiti that spring.) It surely wasn't an easy life, but far more comfortable than the lives in camp nearby, and always a plane ride away from somewhere much nicer. In other words, Penn was living like basically any foreigner in the zone. Nor did many mention Penn's backstory: a trial over an October 2009 altercation with a photographer in Brentwood, California, ongoing when the quake hit. In the spring of 2010, around the time that Oriel died, Penn pleaded no contest to misdemeanor vandalism and was sentenced to three years' probation and 300 hours of community service—the latter servable in Haiti. In the judge's eyes, managing the affairs of an IDP camp home to tens of thousands of earthquake survivors while helping design and implement policy was a form of penance, to be completed while he underwent anger management counseling elsewhere.

In any case, among celebrities, Penn is not alone. When the United Nations Security Council met last month to debate rape in conflict zones, the key testimony came from Angelina Jolie, who unknown to millions of her fans is the real-life Special Envoy for the United Nations High Commissioner on Refugees. George Clooney—that George Clooney—is the real-life chief of a network of spy satellites monitoring troop movements in Sudan. When Congress needed an expert to testify on the current situation of M23 rebel forces in eastern Congo, they called Ben Affleck. In a world where celebrities are leaders, and leaders are expected to act like celebrities, where a quarter of parents say they trust the advice of an ex-MTV dating show host with no medical training on whether to vaccinate their children, and the three most trusted people in the United States are all paid to pretend to be other people on screen, no celebrity need content himself with play-acting an adventurer or “raising awareness” for other people's causes. They can set the agenda themselves.

There is no denying that some of the work by these new celebrity power players sometimes does good. It may even be, as Penn has often argued, that it need not be more pernicious or less effective than those of the “professionals”—after all, career foreign policy and development people have made plenty of messes on their own. But we're all a little too fond of slapping medals or accolades on the famous set, ratifying their half-baked interventions because we think, after years of worshiping in the gossip mags and on the big screen, we know them. If we are going to take these interventions seriously, and consider the real ways in which our culture's power and reach affect people in other parts of the world, we need to take a cue from Mitty, and try to go beyond the image.

Jonathan M. Katz is the author of The Big Truck That Went By: How the World Came to Save Haiti and Left Behind a Disaster (Palgrave Macmillan, 2013), from which this parts of this article were adapted. He was the Associated Press correspondent in Haiti from 2007 to 2011.