Amanda Seyfried

A MUSICAL BENT: Unlike many of her costars, Seyfried felt at home with the format. (Jeff Vinnick / For The Times)

IN THE HBO series "Big Love," Amanda Seyfried plays the eldest daughter in a polygamist family of one husband and three wives. In the lavish new musical "Mamma Mia!," based on the long-running Broadway hit, she plays the only daughter of one mother and one of three boyfriends. ¶ In the movie that features the music of '70s band ABBA as prominently as any of its stars, Seyfried plays Sophie, the daughter of single mom Donna (Meryl Streep), who operates a small hotel on a beautiful Greek island. For Sophie's wedding, Donna invites her BFFs (Christine Baranski, Julie Walters) who were the backup singers of her old group, Donna and the Dynamos. Meanwhile, Sophie secretly invites the three guys (Pierce Brosnan, Colin Firth and Stellan Skarsgard) who may or may not be her father.

Singing was Seyfried's first passion and she trained from age 11 to 17 in her hometown of Allentown, Pa. "When I moved to New York, I quit because I was acting regularly on 'All My Children.' I didn't have time for it."

Although Streep sang in "A Prairie Home Companion" and "Postcards From the Edge," most of the actors were out of their comfort zone. "We were all a little bit freaked out by it," Seyfried says. "I was the least freaked; I knew how to sing and dance and I was basically playing myself in these certain circumstances."

But her three "daddies," she says, were often horrified. "The look on their faces the first day they showed up to dance was just priceless. The facial expressions Colin Firth had were so funny."

The movie, opening July 18, was shot in London and Greece. "It was so beautiful," Seyfried says by phone from the Vancouver, Canada, set of her latest film, "Jennifer's Body," penned by "Juno" Oscar-winner Diablo Cody.

"But it was hectic and tiring. We worked six days a week . . . the entire company partied every time we had a day off."

susan.king@latimes.com