"In many ways with this cast, it’s going to be what I love best about live television, that unpredictable nature," Bergeron said on "GMA."
Skarlatos, Deen, the celebrity TV chef, and reality TV stars/singers Braxton and Zolciak-Biermann were all live in "GMA's" Times Square studio for the announcement.
"This is the first time in a few years that I’ve felt like my weight would allow me to participate," Deen said. "What I hope I’m going to get out of this is a bunch of new friends and maybe lose some weight and a little better body."
Braxton, a star of "Braxton Family Values," a reality TV show about life with her famous sisters, said she had not even told her siblings, including singer Toni Braxton, that she was joining the "Dancing" cast.
"This is the reveal. I haven’t told them yet," Braxton said, adding a, "Hi, girls," shout-out to her sisters.
Skarlatos, 22, an Army National Guardsman assigned to an infantry unit in Roseburg, Oregon, was on vacation in Europe with his friends after a tour in Afghanistan when they stopped a man armed with an automatic rifle and a box cutter who had started firing on a Paris-bound train.
The serviceman said he doesn't "even want to imagine" what his fellow service members think of his turn on "Dancing," but he is excited to get on the dance floor.
"I think it'll be fun. I hope I learn a lot," Skarlatos said. "I'll need to if I'm going to stay on very long."
The four newly-announced cast members were joined by the nine other stars against whom they’ll compete for the coveted mirror ball trophy in season 21 of the popular reality TV dance competition. Those stars -- whose names were announced previously -- are:
Andy Grammer. The 31-year-old singer is best-known for his hit, “Honey I’m Good.”
Victor Espinoza. The jockey, 43, rode the horse American Pharoah to Triple Crown victory earlier this year.
Hayes Grier. The social media savvy 15-year-old from North Carolina shot to fame on Vine.
Alexa and Carlos PenaVega. Alexa, the 27-year-old “Spy Kids” actress, and her 26-year-old husband, a singer with Big Time Rush, will battle each other in their bid for the championship.
Each week the competitors and their pro partners will perform choreographed routines to earn scores from a panel of judges and votes from the viewing audience.
Irwin said she is more scared of the "Dancing" judges than she is of the crocodiles she faces in her native Australia.
"I’m not used to any of this," Irwin said of the reality TV competition. "I’m used to crocodiles so crocodiles are way less scary. You kind of know how they work. The judges? You don’t know what they’re going to say."
Two of Irwin's competitors, Alexa and Carlos PenaVega, are making history as the first married couple to compete on "Dancing."
"A little bit," Carlos replied when asked if he is competitive.
"I didn’t think he was competitive until I was showing him some video footage of what we were learning and suddenly he was like, ‘I really have to step my game up,'" said Alexa.
"Backstreet Boys" member Nick Carter, who joined "GMA" by video from Montana, where he is shooting a movie, said he does not seeing being a member of a boy band as an advantage in the competition.
"Actually it can kind of be a disadvantage in some ways because people probably expect me to be a great dancer," Carter said. "I believe that I’m a good dancer but it’s a different type of dancing so I’ve got my work cut out for me."
One person who may help Carter get the mirror ball trophy is his younger brother, Aaron Carter, who made it to the final five as a contestant on season nine of "Dancing With the Stars."
"My brother is going to try to give me as many tips as he can but I am the big brother," Carter said, with a laugh. "No, at the end of the day I’m open for anything and, you know, we’ll just see what happens."
Actress Rumer Willis, who won the show's 20th season with her pro partner, Val Chmerkovskiy, made a surprise appearance on "GMA" to offer advice to the new contestants.
"I would say the biggest thing is, these pros work so unbelievably hard and if you just listen to them, even if you don't want to, it will be much better for you, I promise," said Willis, who will premiere this month on Broadway in "Chicago." "They work really hard and they're there to support you."
"Dancing with the Stars" returns Monday, Sept. 14, at 8 p.m. ET on ABC.