Jazz guitarist Bill Frisell is about to be outwitted by an inanimate object. With every tap of his finger, a digital delay box to his right is setting off a series of sonic waves, looping and sampling and splintering his guitar lines until it's impossible to tell who--or what--is producing the overlapping tones.

The gizmo seems to be asserting its own will, pushing the guitarist and his band mates into uncharted terrain. Frisell flashes a rueful smile as the music takes another unexpected turn, then tweaks the gadget again to see where the next set of electronic pulses will take him.

By turns brainy and playful, probing and lyrical, this performance at the University of Virginia turns out to be unadulterated Frisell. Which is a good thing: He is perhaps the most distinctive and certainly the most whimsical guitarist to emerge on the jazz scene in the past two decades. His mentor, jazz legend Jim Hall, probably put it best when he described Frisell as a cross between Thelonious Monk and "Far Side" creator Gary Larson.

"I literally laugh out loud when I hear Bill play," says Hall, a godlike figure to Frisell, Pat Metheny and a legion of post-bop guitarists. "He's childlike in the best sense of the word."

Frisell, who appears at Wolf Trap Thursday night with his new quartet--featuring drummer Kenny Wollesen, bassist David Piltch and pedal steel player Greg Leisz--has been extremely busy of late. He has placed himself in a series of challenging settings, willfully blurring the lines that separate genres, surprising himself and a small but devoted following.

In addition to his latest recording, the charmingly bucolic "Good Dog, Happy Man," he can be heard on "The Sweetest Punch," on which he deftly recasts and orchestrates the new songs Burt Bacharach and Elvis Costello recorded on their 1998 collaboration, "Painted From Memory." Frisell's breadth is also revealed by his contributions to new and stylistically diverse albums by saxophonist Don Byron, Mike Stern and Fred Hersch.

In recent years, the 48-year-old guitarist has covered tunes by John Hiatt, Henry Mancini, Bob Dylan, Madonna and the Carter Family, recorded two albums of music inspired by the silent-film star Buster Keaton, and traveled to Music City to record his widely acclaimed album "Nashville" with a lot help from the city's finest pickers. He's also collaborated with Cream drummer Ginger Baker, British vocalist Marianne Faithfull, Living Colour guitarist and Black Rock Coalition founder Vernon Reid, and New York City avant-gardist John Zorn. The soundtrack to the new Wim Wenders film, "Million Dollar Hotel," finds Frisell playing alongside U2 vocalist Bono, rock conceptualist Brian Eno and producer Daniel Lanois.

And on his days off, when Frisell isn't spending time with his wife and daughter at home in Seattle, you might just find him jamming with a certain guitar-playing neighbor, the aforementioned Gary Larson. In fact, shortly after Hall introduced the two, Larson recruited Frisell to compose the music for a "Far Side" TV special. When the project was completed, Larson reportedly deposited a handcrafted guitar on Frisell's lawn. Just his way of saying thanks.

Hours before the show in Charlottesville, seated in the back of the hall during the sound check, Frisell is reflecting on his improbable career and how it took a bashful kid from Colorado on a curious, crisscrossing journey through jazz, pop and country music. He's wearing a black leather jacket that doesn't square with his bespectacled, still boyish face. Frisell speaks softly and slowly, sometimes pausing so long between words that you could fill in the blanks with one of his languid 16-bar solos.

The Costello-Bacharach project is still very much on his mind. He first met the British singer-songwriter when they collaborated on "Weird Nightmare," a Charles Mingus tribute album produced by Hal Willner in 1991.

"We are definitely coming from drastically different backgrounds," Frisell says of Costello. "But in the end, he's so curious about everything and so open. There's no rock star stuff. He seems like one of those guys who's really humbled by music."

A few years ago the two teamed up in London to record a series of duets. But not even that experience prepared Frisell for the task of rearranging Costello and Bacharach's new work.

"The whole idea was that they had these brand-new songs, and the label wanted a kind of 'Birth of the Cool' jazz thing from me," Frisell says, referring to the classic Miles Davis nonet album that drew colors from a palette of jazz and classical instruments.

The first thing Frisell did was to record a tape of the songs on guitar to remove them from their customary setting. "Usually if I'm going to arrange a song, it's something that's been brewing, a song that I've heard most of my life. But I got these songs and had to record them a couple of months later, so I had to internalize them."

Frisell's treatments are often delicately persuasive, displaying his finesse as both a guitarist and an arranger. "I think Elvis really liked what he heard, and maybe he was even shocked by some of the similarities," says Frisell. And Bacharach's reaction? "I haven't heard from him," he says, sounding more concerned than snubbed. "I hope he doesn't hate it or anything."

As with the Keaton and "Nashville" albums, the Bacharach-Costello project challenged Frisell in ways he never imagined. "The tunes are really complex," he explains, alluding to Bacharach's quirky harmonies and meter shifts. "People call it pop music, but there's a lot of weird stuff in there. It's not simple at all."

While recording "Punch" was hard work--"I obsessed over every note," Frisell confides--making the new album was a pleasure for the guitarist, who grew up in Denver playing in lot of local rock bands.

"It was the complete opposite," he explains. "I had these simple little tunes and no arrangements. It felt like I was a singer and this great band was supporting me." In keeping with much of Frisell's recent work, the album projects a fascination with simple but sustained lyricism. "There was a time when I'd repress silly little melodies," he notes. "Now I find myself hoping they'll come out."

Like a lot of his guitar-playing colleagues, including Pat Metheny, John Scofield, Kevin Eubanks and Mike Stern, Frisell is a product of the '70s Boston jazz scene. He attended Berklee College of Music in order to get a grounding in jazz guitar fundamentals, but quickly discovered that he wasn't cut out to play the standard repertoire.

"I think Bill came to Boston to get the jazz side of things together, but believe me, he wasn't a jazz purist," says Stern. "He was into to all kinds of things even back then."

Frisell's first professional break came in the late '70s when he got a chance to record for ECM, the artsy and insular European jazz label founded and run by the autocratic Manfred Eicher. Frisell got the job by default when guitarist Philip Catherine couldn't make it to a session.

"I didn't make much of an impression, " Frisell recalls. "I was just terrified. At that time the label was the ultimate for me. In the early '70s there was just so much that was happening on that label that I really liked. I was really nervous. I could hardly play."

It wasn't long, though, before Frisell was part of the ECM family, recording his own impressive albums and working as a sideman often enough to be viewed as the label's house guitarist. He left the fold in 1987, so that he could have more control over his own recordings.

Being associated with a label that was a jazz genre unto itself had a unique downside, Frisell discovered. "It was a problem at the time because people would say, 'He's one of those ECM guys. I don't like that stuff, so let's not call him.' "

He's since learned to live with labels, albeit reluctantly. "I used to be the ECM guy, and then I was the 'downtown' Zorn guy, and now I'm the 'Americana guy,' " he says, his voice trailing off into a sigh.

The move to Elektra/Nonesuch, Frisell's current Iabel, came a dozen years and a dozen albums ago. "I had to stand up and see what I could do on my own--it was sort of like leaving home," he says. He quickly established his independence on the label with the help of his longtime colleagues, bassist Kermit Driscoll and drummer Joey Baron.

Self-effacing to a fault, Frisell doesn't claim to be an innovator or a visionary, despite a recording history that suggests otherwise. He views himself more as an accidental guitarist, someone who's benefited from a series of fortunate circumstances. He points to the much-praised "Nashville" album, which features musicians from the bands led by Alison Krauss and Lyle Lovett, as an example of his good luck.

Nonesuch label head Bob Hurwitz suggested the idea, he says. "If I was on my own I wouldn't have known where to begin or who to call. I didn't know any of those people. He introduced me to someone who introduced me to all of these incredible musicians. I wish I could take credit for the idea but I can't."

Not everyone has welcomed Frisell's cross-cultural excursions, thematic twists and stylistic leaps. He's probably lost as many listeners as he's gained over the years by following his own muse, and only college radio stations play his recordings with any regularity.

Guitarist Al Di Meola once remarked that critics love Frisell because they understand where he's coming from, but that the public will probably always find him a little too odd, too "wacky" to appreciate.

Frisell shrugs off the comment. "All I can do is what I can do, and I'm not going to change to get more people listening. That's the farthest thing from my mind. But when people come to hear me play, I think I end up touching them. I think I'm getting better at that. I'm not trying to be weird or anything, but whether it is or it isn't, I have to stick with what gets me going."