In the fickle, fast-moving world of rock and roll, it's always nice to be The Next Big Thing. Major labels want you, the press runs to greet you and fans will do anything - yes, anything - to sneak into your dressing room.

But what happens when you're expected to become The Actual Big Thing - not next, but now?

For every artist who makes that jump, thousands fall on their faces. Around 2004 and 2005, a slew of energetic bands such as Kaiser Chiefs, The Killers and Arcade Fire issued explosive debut albums and injected some much-needed energy into the lifeless rock scene. Suddenly, there was more competition for the Next Big Thing crown than fans had seen in quite a while. But just a few years later - a lifetime, in rock terms - many of those acts are releasing second albums that aren't generating the same sense of urgency.

It's called the "sophomore slump," the much-awaited follow-up album that might make - but more often breaks - a promising band. History is littered with examples, from Boston in the 1970s (remember "Don't Look Back"?) to the Spin Doctors in the '90s (chances are you didn't purchase "Homebelly Groove"). Now the '00s are seeing a crop of once-hot bands, from the Arctic Monkeys to The Bravery, facing the pressure of great expectations.

"The sophomore album always seems like the signal as to whether or not this is a band that has enough to make a career, or whether we can start forgetting about them," says Kyle Anderson, assistant editor at Spin magazine. Few bands that score a home run their first time at bat can deliver the goods a second time, he adds. "It's probably an 85 percent failure rate."

It's a syndrome with many causes. One factor is the fans' insatiable desire for something new. Take Evanescence, for example, whose unique sound - a haunting female voice over aggressive rock - helped its 2003 debut, "Fallen," go six times platinum. Its new disc, 2006's "The Open Door," has sold about 1.7 million.

"Maybe that's because they're not the hot new act that everyone's talking about," says Keith Caulfield, an analyst and charts manager at Billboard magazine. Once fans no longer think, "'Oh my God, you're Tori Amos with a rock band,'" he says, "then what?"

Sometimes bands aim for a dramatically different sound and end up shooting themselves in the keyboard. The Killers, for instance, ditched the dance-pop sound of their smash 2004 debut, "Hot Fuss," in favor of the rootsy, grown-up rock of last year's "Sam's Town." (Pretty-boy singer Brandon Flowers even grew facial hair.) Though the album has sold a respectable 1.1 million copies, it also received a thorough beating from critics. And the single "When We Were Young" hasn't exactly dominated radio.

"They lost Top 40, in a way, which was what pushed them to the top," Caulfield says. "They lost their 13-year-old girl fans with the mustache and the spaghetti Western theme."

Another cause of sophomore-itis is plain old exhaustion. Bands who reach stardom early in their career often aren't prepared for the grueling tour schedule, countless interviews and endless meet-and-greets required to keep them in the spotlight. It's no accident that many second albums are downbeat, even bitter affairs, with lyrics about empty hotel rooms, shallow hangers-on and loneliness.

"The problem is, that's not fun and it's horribly self-important," Anderson says. "The pop-punk bands are very susceptible to that. Good Charlotte did the same thing: They had their break-out hit, and all of their songs are about paparazzi now."

Still, it's important to keep success and failure in perspective. The Killers' numbers may have shrunk, but "Sam's Town" is still a platinum album. By contrast, buzz sometimes outpaces sales, making a band seem bigger than it really is. The British group Art Brut, for example, was adored by hipsters and critics but sold only 17,000 copies of its 2005 debut, "Bang Bang Rock & Roll," according to Nielsen SoundScan. And Bloc Party, hugely popular in England, here sold fewer than 300,000 copies of its first album, 2005's "Silent Alarm."

"People think, 'They're huge, I read about them on blogs and there's all these glowing reviews!'" Caulfield says. "The sales numbers [are] in people's minds, they don't really know them."

Here are The Next Big Things from a few years ago, and a look at just how big they might become.

The band: Arcade Fire

The album: "Neon Bible" (Merge)

Then: After the taste-making Web site Pitchfork raved about Arcade Fire's 2004 debut, "Funeral," the Montreal-based band quickly made the jump from underground darling to mainstream phenomenon. Two famous fans named David - one Bowie, one Byrne - helped stir up buzz.

Now: "Neon Bible" debuted at No. 2 on the Billboard chart last month and sold about 92,000 copies in its first week, making it the biggest release in the 17-year history of its indie label, Merge Records.

Next Big Thing? While the new album hasn't created the kind of "must-hear" buzz as the first, any band that can sell out five nights in a row in Manhattan in February and then come back for three more in May - including one at Radio City - clearly has a fan base.

The band: Arctic Monkeys