The Jonas Brothers know their audience.

Throughout the Jonas20: Greetings From Your Hometown Tour, which commemorates Kevin, Joe and Nick Jonas's two decades as a hitmaking trio, they’ve spotlighted surprise guests.

That tradition continued Saturday in New Orleans at a full Smoothie King Center. Nineteen songs in, Nick Jonas embarked on a brief monologue about “how deeply I connected with this (next) song and its message…It’s a dream to perform it with this person.”

With that, Isaac Slade, former frontman of pop-rock band The Fray, took a seat at an upright piano. With Joe Jonas sharing lead vocal duties, they broke into The Fray’s hit “How to Save a Life.” The title track of The Fray’s 2005 debut album, it took on a whole other life in multiple episodes of “Grey’s Anatomy.”

The raptur

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