When banjoist Béla Fleck first met Chris Thile at Merlefest in the mid-’90s, he wasn’t overly impressed. At the time, Thile was a pudgy early-teen, already lauded as a hotshot but still lacking the style and flair that have since made him one of the world’s foremost mandolinists.

“He sounded great, but he was very straight,” says Fleck, recalling Thile’s ability to reel off classic fiddle tunes as a youngster. “I’ve never been good at checking out young players and going, ‘Oh, they’re going to be the next major cat.’ But what Chris has grown into is really something inspiring and amazing.”

Fleck doesn’t shy away from honest criticism, especially with close musical partners like Thile.

“It’s different if they get better than you,” he laughs. “Then you can take pot shots at them when they were young.”

Fleck released his solo debut in 1979 and spent the ’80s touring with Sam Bush in New Grass Revival. His reputation grew with the formation of his band, the Flecktones, in 1988. In recent years, he’s toured with his wife, banjoist Abigail Washington, and with jazz pianist Chick Corea. Fleck holds a record for Grammy Award nominations in the most categories, and he’s taken home a staggering 16 of the gilded gramophones, so far.

Likewise, Thile’s childhood band, Nickel Creek, achieved platinum accreditation with their eponymous Alison Krauss-produced major label debut in 2000, and won a Grammy for their follow-up, This Side. Thile’s third solo album, 2001’s Not All Who Wander Are Lost, features Fleck on banjo. And that’s all music that he made before turning 21.

Today, Thile splits time performing Bach with Yo-Yo Ma, leading his band, Punch Brothers, and filling the giant shoes of Garrison Keillor as the new host of NPR’s A Prairie Home Companion radio variety show.

Given that both Fleck and Thile are subject to “best in the world”-style praise wherever they go, their ability to critique one another becomes an invaluable resource.

“Criticism is a tricky thing,” explains Fleck. “Without it, you can’t improve, but with it, you can really deflate. In America, we’re afraid to criticize and we don’t know how to take criticism. We want everybody to say that we’re great, even when there are things we could do better. But that’s not going to make people go out and really learn the thing.”

Fleck, thus, is as apt a person as any to critique Thile’s performance on A Prairie Home Companion. He compares his affinity for the show to the guilty pleasure of an Air Supply record.

“If I’m driving around on a Saturday, I’m looking for things to do in the car so I can keep it on, because it’s so entertaining,” says Fleck.

But whereas he calls Keillor the “Samuel Clemons of our time,” he feels that Thile doesn’t have the “gravitas” that Keillor brought to the show. “Chris makes up for it by being relentlessly positive, which is how he actually is most of the time, so it’s not a dishonest way to do it.”

Rather than driving around on Saturdays, Fleck has taken to listening to the new iteration of Prairie Home with his four-year-old son, Juno, during extended afternoon baths at their home in Nashville. “I’ve heard some of the shows twice now, and some of them get better with repeated listening.”

Thile’s relentless positivity on stage — including full-body rocking, shaking, and dancing — is the polar opposite of Fleck’s stoic stage presence, where motion is limited to his rocket-fast finger work. Fleck admits that Thile’s restlessness on stage “used to drive me crazy. He would dance around so much that he would throw the beat off when he was younger.”

Now, Fleck appreciates Thile’s “sheer enthusiasm”: “The whole point of being a performer is to be yourself and put that out there, and that’s a big piece of who Chris is. He’s just so darn enthusiastic and excited about life and music.”

Still, Thile’s animation forces Fleck to stand up and sit down throughout the show and to address the audience more. “I’m like the older cat. It’s lame if he’s going to jump around and be this great emcee and talk all night, if I just sit there. I’ve got to be involved. So we move around and then get serious when it’s time to play the music.”

The music at a Fleck/Thile show includes songs that fans of each may have never heard. Before a similar one-week tour last year, the duo got together for three days and wrote 20 “very complicated pieces,” perfecting them individually during daily practice sessions on the road.

“We conceptualize ideas very quickly together and are comfortable jumping off of each other,” says Fleck, explaining that even when they haven’t worked out a set ending to a piece, they’re able to improvise it to a conclusion together.

Outside of the original works — which they’ll debut more of in the six-day tour that begins at the Music Hall this Tuesday — they’ll pull from each of their extended catalogs and favorite covers.

“The show is easy to put together because Chris can play anything I know, whether he’s played it before or not, and then we choose things from his repertoire that I have a good shot at,” says Fleck.

Fleck’s post-Thile summer includes a run to Europe with Corea, dates with the Flecktones, and a new album with Washburn that will be released in October. But he says that his partnership with Thile is still budding, and that the songs they’ve written together will continue to be honed over the next “year or two.” Most of all, performing with Thile is an opportunity for Fleck to be challenged in intense spurts, without having to leave his family for more than a week.

“I’m going to get my rocks off musically with Chris. I need these jolts of high musicality from another angle,” says Fleck. “The conversation that we have when we’re playing is very spirited and spontaneous, and it’s a lot of fun. A lot of fun.”

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