It's no secret that Britney Spears is somewhat bizarre when it comes to her behavior, but she's not the only high-profile performer who swings from smiles to silence.

Pop Tarts popped itty-bitty beauty Mary-Kate Olsen at the Park City premiere of her new flick "The Wackness" on Friday.

The typically media-silent screen star didn't talk while walking the red carpet, but for a girl who always hides from the photogs, she did hang out on the press line for an extra long time to have conversations with her co-stars, creating an ideal photo op.

Hmm — could it be that MK doesn't mind being sold off to the weeklies as much as we may have thought?

The starlet was even happy to accommodate the request of a snapper who came late by posing on the platform with a (slight) smile. But just minutes before that, it was all frowns for the flashes.

"It was hard to get Mary-Kate to smile," a snapper told Pop Tarts. "She is very unpredictable. One minute she was smiling and laughing along with her co-stars, the next thing she was totally somber. It was like a lottery whether we could catch her looking even a little happy."

But like our Brit, this tiny twin is a multimillion-dollar mogul who also doesn't seem too concerned with dressing just for impressing. The 21-year-old strutted without blush or a hairbrush — but when you're one half of your own uber-famous franchise near the top of Forbes' Under 25 Rich List, there probably aren't too many peeps you need to win over anymore.

That said, MK did do a good job of winning over "The Wackness" audience, proving that she can show a little lovin', even if it is just in reel life. "The Weeds" sensation plays a stoner called Union who makes out with Ben Kingsley, 43 years her senior, in a steamy 20-second lip lock. How, ah, sweet.

Later that night, Pop Tarts popped Olsen partying at 5W and Butter nightclub's private Park City party.

MK pulled a Britney and changed her outfit and looked particularly cute this time with fresh makeup and a combed head of hair.

The Tinseltown tiny sipped some adult beverages and lounged around alongside other stars like the Hilton heiresses, Cisco Adler and Kim Kardashian. (Kimmy left as soon as her former friend Paris Hilton showed up, apparently to avoid any potential problems.)

So even though Olsen isn't mad for media attention, it seems the "real" Mary-Kate is a "free-spirited" star.

"She is an amazing professional and just has such a free spirit that draws you close to her," Kingsley told Pop Tarts.

Bono Rejected From Park City Party

Here's a lesson to be learned by all — regardless of how much money you have, how much music you've made or how much of a humanitarian you are, sometimes you just can't have a good time with your guys.

According to insiders, Bono turned up to the bash for Butter/5W PR in Park City on Friday night, but was booted at the door. Twice.

"First of all, he came with eight guys but was told that was too many guests," said our source. "He then came back a little later with just five but they still didn't let him in."

Ouch. But true to New York nightspot Butter's rep, it was one meltingly mad party (even without the world's most wondrous man).

Tisdale and Hudgens Get Huffy and Puffy

With her boyfriend Zac Efron recovering from appendicitis in the hospital, it seems Vanessa Hudgens is spending quality time getting sweaty with someone else — her "High School Musical" co-star Ashley Tisdale.

The Hollywood hotties have been busted several times this week hightailing it into Pilates classes at their local gym in Encino.

But we do hope Efron recovers soon just so he can assist his girlfriend with her wardrobe, as we aren't too sure what to make of her workout attire.

Click here for a pic

But at least we know now how Hudgens and Tisdale remain so taut and tiny. Both gals recently told Tarts that their exercise regime consists of "Pilates, Pilates, Pilates" and they swear by the formula for keeping flab-free.

Maroon 5 Boys 'Over It'

Things certainly have changed for what was once a little indie film fest, seeing as though the likes of Maroon 5 and a number of other big-time bands were making music just as massive as the movies at Sundance.

After spending some of the day scoring free swag at a suite, the boys got down to business at their sellout gig at the MySpace/Tao lounge.

But that business didn't involve opening their mouths. After posing for a quick pic on the press line, the guys grunted away — an insider telling Tarts that they were "difficult" and "said they refused to do interviews because they were over it."

Their performance, meanwhile, was temporarily delayed when Park City's power supply decided that it was "over" the sudden overload of people and blacked out on Friday night.

Thankfully, there were a few other celebs at the concert that weren't too precious to promote. The former wife of Rod Stewart, model Rachel Hunter, rocked out in jeans and a cap — but didn't want to be seen with "hat hair" when asked if she could remove it on the red carpet.

But Rach was comfortable crooning about revealing a little more down lower.

"I'm really focusing on the branding aspect and my swimwear line that I'm launching at home in New Zealand," she said. "But I never make New Year's resolutions — I don't put that kind of pressure on myself."

Meanwhile, her "good friend" Ian Ziering also slummed with her into the snowy city and stood slightly awkwardly as everybody hounded Hunter, leaving him to at least try and look busy.

But finally, the former "90210" boy had his chance to shine, and Pop Tarts was pleasantly surprised by his style.

"Yes, yes, one day I do want to see something I've directed or produced to be here at Sundance," Ziering (who has created, directed and produced his own series for MySpace) told us.

"And I'm going to be back on TV soon myself, something exciting is happening this year. Finally."

Spotted or Snapped a Star? Got a Good Scoop? E-mail poptartstips@gmail.com.