"I don't believe in stepping out front on matters that you're just getting educated on," says Edward Norton. "I gravitate toward topics on which I have something to contribute." For Norton, a lifelong environmentalist whose father, Edward Sr., was the senior adviser for the Nature Conservancy's China program, that's meant throwing his weight behind a litany of issues, like the greening of affordable housing. While growing up in Chesapeake, Maryland, Norton volunteered at the Enterprise Foundation, a nonprofit started by his grandfather, the philanthropist James Rouse, that provides housing for low-income communities. Norton, who sits on the board of trustees, convinced Enterprise to start the Green Communities Initiative, a $550 million commitment to make affordable housing more sustainable nationwide. In 2003, Enterprise partnered with the energy company BP Solar to launch its Solar Neighbors program, which donates a full solar setup to a low-income Los Angeles family for every solar system a celebrity buys.

At home, the Manhattan-based Norton is a founding board member of Friends of the High Line, an advocacy group working to establish a public park on an abandoned train line on the city's West Side. "Creating a green space in Manhattan isn't a high-crisis issue," says Norton, who doesn't own a car and takes public transportation. "But you have to tune in to what's relevant in your community." Next up: Hollywood. While filming The Incredible Hulk: Part 1 in Toronto this fall, Norton (who bulks up in a digitally engineered green suit for the role) attempted to reduce the film's footprint by cutting back on paper waste, eliminating idling vehicles on set, and exploring the development of a film-production carbon calculator. "I wouldn't say we've achieved any meaningful success yet," he says. "But sometimes you just have to put one foot in front of the other and start."