An encounter in Africa changed Matt Damon, he tells Dorinda Elliott, who finds that modesty becomes the superstar

MATT DAMON'S BIG IDEA—THAT EUREKA moment during a journey when you see something or have a conversation with someone and you are, for an instant, touched and in some way transformed—came in Zambia. He was there in 2006 with DATA (Debt AIDS Trade Africa), the rock star-activist Bono's organization, wondering how he might put his own celebrity muscle to the best use.

Damon had already visited AIDS clinics and urban slums, and on this particular day, he was in a small village, trudging in the blazing sun alongside a young Zambian girl. As they marched to the village well, she told him proudly that she walks to school, three miles each way. He asked if she wanted to live in her village when she grew up. "No!" she said. "I want to go to Lusaka and become a nurse!"

Driving off a while later, Damon reflected on the moment and concluded that it was the village well which gave the girl permission to dream. "If someone hadn't put the well there, that girl would be spending her entire day trying to find water just to survive to the next day. You could forget about her going to school. That well gave her a future," he tells me. "I remembered as a kid sitting with Ben Affleck, dreaming of someday going to New York and becoming actors. Dreaming is a great thing."

Damon and I are in the restaurant at the Crown Plaza Hotel in Springfield, the empty, windswept capital of Illinois, discussing world poverty over a couple of Starbucks lattes. No one is paying any attention to us, and that seems to be the way he likes it. There are no bodyguards here—Damon's beefy security man has already checked out the quiet restaurant and disappeared—and no fans in sight. Dressed in his signature well-worn baseball cap and a hoodie sweatshirt, Damon is ready for another day of filming The Informant, a Steven Soderberg production about a whistle-blower at Archer Daniels Midland who exposed price-fixing in the 1990s. But the actor is clearly delighted to be talking not about Hollywood but rather about global poverty, water, and Africa.

Damon is as self-deprecating as you might expect from the soft-spoken roles he has played—from the troubled kid in Good Will Hunting, to the nerdy member of the Ocean's Eleven team, to the amnesiac being chased down by the CIA in the Bourne films. Celebrity, he says, is "a tricky thing to navigate. There's no real pretty way to do it." He is also acutely aware of the power—and the pitfalls—of fame when it comes to shining light on international issues. Damon has wrestled with everything from not wanting to be used for the wrong causes to the fear of coming across as dumb. "For a lot of actors, our biggest fear is that we're going to start talking about things we don't fully understand and sound like idiots," he says. "In the long run, I'll do much more good if, when I open my mouth, I have something worth saying."

And so Damon set out to educate himself, traveling to South Africa and Zambia in April 2006. As a subject, poverty and Africa "seemed daunting, and there's so much to learn," he tells me. "You have to give yourself permission to not know. It's a long process."