Anytime Blues Traveler is mentioned, a picture immediately comes to mind: a large man in a floppy, army-green fedora and a vest laced with as many harmonicas as there are keys on a piano.

John Popper is the face of Blues Traveler. Though he is much smaller now, without that vest and with a crisp brimmed number traded for the fedora, he still reigns as the extraterrestrial harmonica frontman of a band that continues to be as prolific as it is versatile, even after 20 years.

On Blues Traveler’s new album, North Hollywood Shootout, the last song, “Free Willis, Ruminations from Behind Uncle Bob’s Machine Shop,” features an unlikely surprise guest vocalist: Bruce Willis.

Willis grittily belts out, “Would like to sit here and talk about it? A man doesn’t like to be held down… ”

The Die Hard star’s Charles Bukowski-esque improvisational prose hovers over dissonant instrumentals reminiscent of early Velvet Underground. The result is an innovative exodus that is representative of the entire album’s unorthodoxy.

The band’s 10th studio album, produced by the Grammy Award-winning David Bianco, is named after their recording experience in North Hollywood near the site of where a gunfight had once occurred.

“It was a different process because we wrote the songs while we were in the studio,” says bassist Tad Kinchla, before a sound check in Pittsburgh, Penn. “We came up with all the songs together and recorded them on the fly.”

Though the band had a lot of new songs already written when they entered the studio, they came to an agreement to write all new material together, as they recorded the album.

As for the Willis cameo, Kinchla explains that actor has been a long-time fan and friend of the band, so one night they asked him to sit in on a song and improvise lyrics.

Shootout also features some deeply personal tunes. The track “Forever Owed” was inspired by Popper’s recent USO trip to Afghanistan and Iraq; “Borrowed Time” is about the recent passing of Chan and Tad’s father.

The Princeton, N.J., quintet’s two-decade lifespan has been the result of their ability to consistently produce appealing original material while always taking musical risks (as demonstrated by giving complete lyrical liberties to Willis).

Blues Traveler first became nationally known in 1994 for the happy-go-lucky anthem “Runaround,” from their album Four. The song was the longest-charting radio single in Billboard history. Other singles like “Hook” and “Most Precarious” also climbed charts, but their musical creativity is best displayed in their live performances, where they take 15-year-old, four-minute songs to new 10-minute interpretations every time out.

In the early ’90s, when a new generation of “jam bands” like Phish and the Dave Matthews Band became popular, Blues Traveler joined the party and started building a larger, loyal base of fans who traveled many miles to their shows night after night to see their extended, blues-driven jams.

To reciprocate the love for their fans and the bands that embrace the art of extended jams, Blues Traveler started the H.O.R.D.E. (Horizons of Rock Developing East Coast) Festival in 1992. In its seven years of activity, everyone from Neil Young to G-Love and Special Sauce has joined the festivities.

But in 1999 the band’s fate was unexpectedly put in jeopardy when bassist Bobby Sheehan died from a drug overdose. The remaining members decided that Sheehan would have wanted Blues Traveler to continue so Tad Kinchla, the brother of Chan, the band’s guitarist, was asked to join. Keyboardist Ben Wilson also joined the band at this time.

Instead of trying to recreate Sheehan’s style of bass playing, the band let Kinchla bring his own vibe. A graduate of Brown University who majored in urban studies, Tad can’t imagine doing anything else with his life except playing bass for Blues Traveler.

“This work is tiring but very rewarding,” he says.

This past summer, they played nonstop, touring and performing at festivals like Lollapalooza and Austin City Limits.

After years of concerts, the band thrives on not knowing where their music will go next during their live shows. Every night they play a new set list written by a different member of the band.

“Playing this way makes performing every night still fun and interesting,” Kinchla says.

BLUES TRAVELER plays 8pm Wednesday, Nov. 19, at the Golden State Theatre. 417 Alvarado St., Monterey. $29, $39, $59. 372-4555.