Irish rockers U2 stood astride the music world as the 1980s gave way to a new decade. What casual fans couldn’t know was how close the band was from becoming, in the words of lead singer Bono, one of music’s biggest clichés.

They were talking about breaking up over “artistic differences.”

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The new Showtime documentary ‘From the Sky Down,’ debuting at 8 p.m. EST Oct. 29, recalls how the band’s 1991 album ‘Achtung Baby’ restored their faith in each other while cementing their rock god status.

The film may not convert those immune to the band’s arena rock anthems or those who find their socially conscious pose hypocritical given their affinity for tax shelters.

Frankly, director Davis Guggenheim (‘An Inconvenient Truth,’ ‘Waiting for ‘Superman”) isn’t interested in expanding the band’s fan base nor exploring universal themes. It’s a portrait of a band in crisis, one which focuses like a laser on how the U2 sound came to be.

‘From the Sky Down,’ which opened the Toronto International Film Festival, begins with U2 rehearsing for a Glastonbury concert to feature songs from ‘Achtung Baby.’ It’s the Spring of 2011, and the rockers are in a reflective mood.

“There is a moment when it’s dysfunctional not to look back at the past,” says Bono, a veritable fountain of tasty sound bites.

So Guggenheim flips the calendar back to the band’s earliest days, when a simple pub band from Dublin first hit American shores. We see more than an act conquering a nation. We watch as the quartet embraces America, both its people and musical spirit. The latter, culminating in the disappointing concert film ‘U2: Rattle & Hum,’ shook the band’s confidence.

“You start to believe what people are saying about you,” Bono recalls of the film’s mediocre notices. “Maybe this is the end.’

The band eventually regrouped and, inspired by the collapse of the Berlin Wall, shook up their formula the best way they knew how.

“Making ‘Achtung Baby’ was the reason we’re still here now, the pivot point. Either we’re going forward, or this is the moment when we’re going to implode,” Bono says.

‘From the Sky Down’ may be one of the best X-rays of a band’s musical process. We watch group members noodling over the smallest song details, hear Bono scat singing in rehearsal and watch as musical keys float on the screen.

That minutiae, initially fascinating, starts suffocating the film’s entertainment value. Even diehard fans might beg for another irresistible Bono quote to bring some humanity back into the narrative.

Far better are scenes debunking the band’s oh-so-serious image. We watch Bono and co. strike rock star poses for ‘The Joshua Tree’ album cover against their better judgment, mumbling about how silly the whole process feels. And the film’s detour to Berlin to witness history proves both practical and a singular inspiration.

‘From the Sky Down,’ part of a massive U2 repackaging of ‘Achtung Baby’ coming Nov. 1, is a purist’s delight. With a little tweaking, it might have gone down as one of the more instructive rock documentaries of the modern era.