From the album Is Joy Easy
This is a song about watching someone perform joy while convinced you're experiencing it together. The narrator marvels at Mary's power and presence, describing her charm as something she practices and turns on like a switch. What he thinks is mutual connection is actually one person working and one person watching.
Mary's in her dress / It's getting dark / Sometimes blending in / You just play your part
Playing a part means performing a role you don't choose. The song opens with Mary already at work before she even arrives.
She waves into the mirror / And then she smiles / She's practicing her charm / Before she turns it on
Charm you practice in private and turn on in public is not spontaneous joy. It's rehearsed labor, and the narrator sees this preparation but still mistakes the performance for the real thing.
Well, she walks into your party / And she's lookin' for somebody / And the music turns her / 'Round and 'round
The music turns her, not she turns to the music. That single word choice suggests she's being moved rather than moving, acted upon rather than acting. The narrator might not notice the difference.
Could life be one plus one? / When you grab my arm / And dance to this song
He asks if connection could be additive while describing her grabbing his arm, an action that could read as affection or obligation depending on whose story you're in. The question stays a question because he's not sure what's actually happening.
I said, 'Wow' / That girl, she got power
Power is what you call it when you don't understand how something works. He reduces her labor to a magical quality she possesses, which lets him keep watching without asking what performing joy might cost her.
The song keeps asking if joy is easy while showing Mary working to produce it. The narrator sees her practice, sees her turn it on, watches the music move her, and still wonders if what he's feeling is connection. That gap between what he describes and what he believes is the whole song.