It wasn’t always the case. Back in the Potter days, long before she worked with a stylist, Watson would raid her mother’s and her stepmother’s closets for premieres. “They just didn’t design stuff for 11-year-olds,” she says, laughing. “There’s some stuff I think, ‘I never should have worn that!’” When I mention the 2001 Harry Potter and the Sorcerer’s Stone premiere, she almost spits out her apple juice. “It was a long, denim Kenzo dress, and these crazy fake-python boots. I loved it! You wouldn’t expect an 11-year-old girl to wear that. But I’ve always been adventurous.”

Watson is talking about fair trade, a subject she studied while at Brown University, Rhode Island (where she enrolled in 2009), and the cause she is pouring her heart into, the Green Carpet Challenge (GCC). Eco campaigner Livia Firth began the GCC in 2009 to catapult ethical fashion into the global spotlight.

For Firth’s latest project, she asked five British-based designers to each create two event pieces exclusively for NET-A-PORTER. But who to showcase them? The two women had met previously at a party, so when Firth asked Watson to join the project, she jumped aboard. “I was like, ‘No one’s doing anything like this!’ It’s so exciting.”

Having grown up on the red carpet, Watson knows first-hand that sustainable style and glamorous gowns have never been totally simpatico. “I’ve always had this huge problem,” she says. “I would love to wear garments that are ethically sourced, but there aren’t enough options for me to be able to do that realistically.” So when she met Firth, “it just seemed like [the project] was something I had to do, something I’d been waiting for.” Her eyes light up: “Livia’s created a lobbying body to put pressure on governments and corporations to encourage them to have [ethical responsibility] as their baseline. It’s quite awesome.”

The GCC isn’t Watson’s only fashion-related passion project. In 2010, she designed the first of three collections for Fairtrade brand People Tree. In 2011, she used her expertise to create a sustainably produced capsule collection with Alberta Ferretti, Pure Threads. The projects were well received: “Though they were tiny collections, they seemed to raise awareness and a groundswell of interest,” Watson says.