With uncharacteristic humbleness, Bono — U2’s bombastic yet charismatic frontman — speaks softly.

“As an artist, your biggest enemy is your own history,” he says, and that’s that’s the crux of Oscar-winning documentarian Davis Guggenheim’s film “From the Sky Down,” which peels away U2’s public image to get at the band’s private reality, with talking-head interviews and rare archival concert and rehearsal footage from the U2 vaults.

Premiering tonight on Showtime, Guggenheim’s film is framed in interview format, where Bono and his band mates — guitarist the Edge, bass player Adam Clayton and drummer Larry Mullen Jr. — look back 20 years to when the quartet struggled to reinvent their sound for “Achtung Baby,” a record that has become recognized as one of rock’s greatest.

The documentary is tied to the 20th anniversary of the release of that album next month. It’s an all-access pass behind the scenes at this pivotal moment in the band’s history, when the Irish quartet distanced itself from the Americana of their early hit albums like “The Unforgettable Fire” and “The Joshua Tree.” They hoped to create a musical mutant that combined arena rock with emerging beats of European club culture and the abrasive industrial-style sound popular in Germany.

The mass appeal for this narrow slice of music history will probably be limited to U2 fans, but that’s still a pretty huge audience, considering that the band has sold 150 million records and earned 22 Grammys.

With rare black-and-white clips, the documentary quickly recounts the band’s rise from local Dublin heroes to rock stars able to fill stadiums. And that’s where things get interesting.

The film is filled with surprises that many fans will see as revelations, such as Mullen’s steely hesitance to share percussion duties with a computer drum program, and how the Edge was literally at the edge — distraught over his dissolving marriage (chronicled in the song “Love Is Blindness”).

There are also scenes where good-natured do-gooder Bono is caught in an angry rant about a miscue by a roadie — an insight on just how tightly wound U2 was during this period.

Hands-down the most compelling segment is when Bono and The Edge listen to an old rehearsal tape of a song called “Sick Puppy.” It’s a glimpse into how U2 makes music, as the guitar riffs by the Edge emerge into what would eventually become the hit “Mysterious Ways.” They do it again with the band’s famous tune, “One,” a song born out of an Edge guitar improv and Bono’s scatting nonsense.

In fact, had there been more insights on how the individual songs of “Achtung Baby” were created, the documentary would have benefited.

But, what this film does capture nicely, is the band’s camaraderie, even during difficult times.

It also shows off Bono’s humor, not always apparent when he’s preaching social activism.

At one point when he speaks about developing a stage character dubbed “The Fly” — who’d represent the new U2 sound — he jokes it’s a rock star borrowing Lou Reed’s sunglasses, Jim Morrison’s leather pants and Elvis Presley’s jacket.