For U2, the end of the '80s spelled incertitude.

Their 1988 album Rattle and Hum performed well commercially, but had been met with disappointment from critics. One Rolling Stone writer described the LP as "misguided and bombastic." Another from the Village Voice put it more bluntly: "By almost any rock and roll fan's standard, Rattle and Hum is an awful record."

Dismayed but not deterred, U2 was left wondering what to make of this stiff feedback. "It was a very sobering, challenging period for us — very uncomfortable," the Edge told Q magazine in 2011. "The band got too straight in the '80s," lead singer Bono agreed. "I mean we were wrestling with some pretty big questions."

It was clear a drastic change was in order. U2 headed to Berlin, Germany, a city that by then had hosted numerous other artists in search of their own growth and revelation. "We have to go away and dream it all up again," Bono told the crowd on Dec. 30, 1989 during a concert in Dublin.

They tucked away inside Hansa Studios with returning producers Daniel Lanois and Brian Eno, then began recording sessions for U2's next album, Achtung Baby. An initial demo recorded there ended up becoming the inspiration for three separate songs: "Ultraviolet (Light My Way)," "Zoo Station" and the album's first single, "The Fly."

"One day, [engineer] Flood had a different look in his eye," Bono recalled for 2006's U2 by U2. "It started to feel good. We recorded 'The Fly.' Edge's guitar sound was literally like a fly had broken into your brain and was buzzing around." (This sound was achieved by mixing an additional guitar on top of the existing guitar in the opening, resulting in a phasing type effect.)

As sessions moved back to Ireland, Bono was also embracing his eccentric side. He began developing a character nicknamed "The Fly," donning a pair of buggish, comical-looking sunglasses to the amusement of his bandmates. The result was an emphasized version of the egomaniac some had accused him of becoming.

At the same time, Bono had "became very interested in these single-line aphorisms," Bono said 2005's 1000 UK Number One Hits. "I had been writing them, so I got this character who could say them all, from 'a liar won't believe anybody else' to 'a friend is someone who lets you down' – and that's where 'The Fly' was coming from."

Watch U2's Video for "The Fly"

Bassist Adam Clayton recognized that experimenting with an alter ego, as well as with new studio techniques, was an exciting endeavor. But it was also a risky one. "At that time, it was impossible to know whether U2 fans would follow Bono down this particular path, so ['The Fly'] was a real leap of faith," Clayton says in 1000 UK Number One Hits. "The whole track is a high-energy sonic barrage, but with an angelic chorus. It's a classic example of U2 and Eno interfacing."

Making rock music, they surmised, didn't need to be so serious. "One of the lines that didn’t make it into the song 'The Fly,' one of the clichés that we developed, was that 'taste is the enemy of art,'" Bono told Rolling Stone in 1993. "There’s a point where you find yourself tiptoeing as an artist, and then you know that you’re in the wrong place. It’s like you have a rule book, but you don’t remember where you got it.

"And along with that being true of the music, it can become true in a wider sense," Bono added. "I felt like I didn’t recognize the person I was supposed to be, as far as what you saw in the media."

Bono's new character was first introduced to the world via the music video for "The Fly," and then on stage in person for their Zoo TV Tour, which continued through 1993. "I always thought of the Fly as a meltdown kind of a guy," Bono said. "You gotta find new ways of saying the same things, you really do. ... I think it is very surreal, and it was amusing to us even then. We were aware of how ridiculous it was."

However ridiculous, "The Fly" was received favorably by critics who had previously dismissed Rattle and Hum, clearly impressed by the harder, edgier sound, combined with more vulnerable lyrics. The song did well on modern rock radio and became U2's second No. 1 single in the U.K., following "Desire" – which ironically came from Rattle and Hum.

"We're the most loved and the most hated band on Earth," Bono said to Q magazine. "A lot of the reasons people don't like us — apart from myself, which I understand because I have to live with me too — are actually what make us interesting. It's the tangents, the explorations, the making mistakes in public.

"But U2, love or hate them, all the things that shape and form the time – maybe not the fashionista, but all the other things – we're right there. I hope that that makes it an interesting spectator sport."

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U2 don't inspire weak reactions in people. There are passionate U2 fans, and passionate U2 haters, and very little in between.