Mary-Kate Olsen may be pegged as a fashionista by the press, but the actress says her favorite look of all is her birthday suit.

“I run around my house naked with heels all the time,” Olsen, 21, tells Harper’s Bazaar for its October issue. “It’s so funny. All my friends will tell you I love running around in kimonos and jewelry or naked with jewelry.”

In public, the actress has famously cultivated her own unique look, drawing from vintage-inspired fashions. ...