If you're going to watch anyone sleeping in a raised glass case, it might as well be actress Tilda Swinton, whose fey and androgynous beauty lends itself well to an act that's fairy tale-like in nature. Swinton has spent two full days this past week dozing—at least ostensibly—in a translucent box in a gallery at New York's Museum of Modern Art, as part of a performance piece titled The Maybe, which she first presented at London's Serpentine Gallery in 1995. While Sleeping Beauty's glass coffin was watched over by seven dwarves—or at least that's how it went in the Disney movie—Swinton was observed on Monday by visitors who chanced upon her performance in the back corner of a second-floor gallery.

"She's so skinny!" gasped a young man named Reed, who later identified himself as a medical student. "Do you think she can hear us?" one girl asked another after a long, involved conversation about whether or not Swinton was sexually attractive. They had decided in the negative. If she did hear them, the actress didn't react; rather, she stayed curled up on her side on a white mattress, her tiny body fully clothed in jeans, a plaid shirt and canvas sneakers. Almost imperceptibly, her chest rose with each breath. Underneath her head, which was crowned by a spiky mop of platinum blond hair, she tucked her left hand; this tender, innocent gesture only served to make her look more otherworldly.

The crowd, their heads tilted, clustered around her face, which was elevated to eye level. Light from a video work in the same gallery, Douglas Gordon's Play Dead; Real Time (2003), illuminated her taut skin, with its bluish sheen. The Gordon piece showed footage of elephants lying down and getting up again in a white room—juxtaposed against it, it was hard not to see Swinton as another sort of captive animal. "Is she real?" one man asked his girlfriend. "Or is she made of wax?"

I overheard two girls talking about how Swinton had changed positions at one point, rolling over from her back to her side. "How long have you been here?" I asked them. "One and a half hours," the first responded. They weren't sure if Swinton was actually sleeping, but when I asked them if they thought she had taken sleeping pills, they seemed scandalized. "That would be such a farce," said the second, whose name is Sunny. "Why do a performance piece if you're going to fake it?"

If you'd like to guess for yourself, you can try your luck at MoMA, where she's scheduled to perform multiple times in the coming weeks—the museum, however, has not released times or dates.