'Dancing With the Stars' Contestants Size Up Competition Before Premiere

What are the dozen TV, music and sports stars that comprise the new celebrity cast of  " Dancing With the Stars" doing in the hours before they make their debut tonight?

Practicing, practicing and practicing.

As the celebrities have been cramming away in rehearsals to prepare for their debuts,  " Good Morning America" got an exclusive look inside the "Dancing With the Stars" ballroom as the stars hit the actual ballroom floor for the first time.

The action on Sunday revealed that the stars are everywhere from in panic to in pain, but none are taking the competition lightly.

Watch the two-hour season premiere of "Dancing With the Stars" tonight,  Monday, March 19, at 8 p.m. ET on ABC.

"I'm having panic," singer Katherine Jenkins said after taking a spin on the ballroom floor with her partner, Mark Ballas.  "I am a classical singer.  I make albums and do sort of classical concerts."

Her fellow singer in the group, soul legend Gladys Knight, said that she too was having trouble adjusting to performing with her feet, not her voice.

"I feel a little bit, uh, out of my league because I don't have a mic," Knight said.

PHOTOS: Behind the Scenes of the New 'Dancing With the Stars'

Contestant Sherri Shepherd talked about the aches and pains that the hours of rehearsals with her pro partner,Val Chmerkovskiy, have caused her.

"My back, my toes, my belly," Shepherd said when asked which part of her body hurt the most.  "Did I say my back?"

Over the weekend "The View" co-host tweeted that her pain is even affecting her fashion choices.

"Went to put on my 5? church heels, but my feet are hurting so badly, @ SalfromtheD made me put on my tennis shoes," she wrote.

Some Season 14 cast members are already revealing the benefits of all the rehearsal hours - the instant weight loss  that has become a hallmark of the reality competition.

35-year-old Jaleel White, who famously portrayed "Urkel" on the 1980s sitcom "Family Matters," says he's already lost 10 pounds.  Telenovela superstar William Levy, known as "the Brad Pitt of Mexico," says he has danced himself 12 pounds lighter practicing with pro partner Cheryl Burke.

While perfecting their own Cha-Cha and Foxtrot steps, the stars were not above keeping a close eye on their competition.

Gathered around iPhones, iPads and monitors during Sunday's rehearsal, at least one couple was willing to admit the other couples they most fear.

Jaleel White, William Levy and Sherri Shepherd were named by former "Little House On the Prairie" star Melissa Gilbert and veteran "DWTS" dancer Maks Chmerkovskiy as likely picks for the final three.

"We can't underestimate the 20-year-old kid who can do the standing back-flip either," said Gilbert, referring to Disney Channel star Roshon Fegan, who is paired with pro dancer Chelsie Hightower.

Actor Jack Wagner, NFL pro Donald Driver, tennis legend Martina Navratilova, singer Gavin DeGraw and "Extra" host Maria Menounos will also compete on the dance floor tonight.