My favorite artists are the restless ones — those who enjoy, even thrive on, working outside their comfort zones, refusing to be confined to any convenient pigeonhole.

Banjo magus Béla Fleck might be the poster boy for such musical curiosity. The 18-time Grammy-winner is fluent in bluegrass, newgrass (which he helped define), jazz, rock, Indian classical and various other traditions from around the globe.

Opera superstar Renée Fleming also enjoys a boundary-defying inquisitive streak. While she's one of the finest, most celebrated, sopranos of her generation, Fleming has stepped into surprising places, including a Super Bowl performance, recording a moody rock album sporting songs by Peter Gabriel and Tears for Fears, singing Broadway and jazz, and now, with Fleck, performing old Appalachian folk songs behind an office desk. What will she do next?

The seemingly odd bedfellows have surrounded themselves with an A-list cast of bluegrass musicians, including mandolinist and singer Sierra Hull, bassist Mark Schatz and guitarist Jake Stargel. Of special note is Michael Cleveland who, arguably, is the third singer in this band. His warm, perceptive fiddle provides commentary, harmonizes with Hull and Fleming and occasionally steps out beautifully on its own.

These songs speak of longing, of grief during war and the sturdy hope for better times to come. They may be a couple hundred years old, but they resonate today, especially in these bittersweet renditions served up by two ever curious, like-minded artists.

SET LIST

• "In the Pines"
• "He's Gone Away / Storms Are On the Ocean"
• "Blackest Crow"

MUSICIANS

• Renée Fleming: vocals
• Béla Fleck: banjo
• Michael Cleveland: fiddle
• Mark Schatz: bass
• Sierra Hull: mandolin, vocals
• Jake Stargel: guitar

TINY DESK TEAM

• Producer: Tom Huizenga
• Director/Editor: Maia Stern
• Audio Director/Mix: Josh Newell
• Host/Series Producer: Bobby Carter
• Videographers: Maia Stern, Joshua Bryant, Kara Frame
• Audio Engineer: Neil Tevault
• Photographer: Elizabeth Gillis
• Tiny Desk Team: Ashley Pointer, Felix Contreras 
• Series Editor: Lars Gotrich
• Executive Producer: Suraya Mohamed
• Executive Director: Sonali Mehta
• Series Creators: Bob Boilen, Stephen Thompson, Robin Hilton