Mamma Mia! Only ET is with the stars of this summer's most anticipated musical, and only ET's Jann Carl is soaking up the sun in sunny, picturesque Greece with Pierce Brosnan and Amanda Seyfried!

"I was nervous; [but] when I actually got down to it, it was very easy in some respects," Pierce tells Jann about having to sing on camera. "I spent six weeks listening to my iPod and driving my kids nuts and my wife nuts and the family and the dog and everybody nuts, you know, just singing these songs."

In theaters July 18, 'Mamma Mia!' stars Meryl Streep as an independent, single mother whose spirited daughter (played by "Big Love" star Amanda) is getting married -- and is determined to learn the identity of her true father so he can walk her down the aisle. Pierce, Colin Firth and Stellan Skarsgard are the three contenders, all of whom were involved with Meryl's character 20 years earlier when they visited her Greek island paradise. Inspired by the incredibly catchy music of Abba and based on the hit musical that toured the world, 'Mamma Mia!' features such sing-along classics as "Dancing Queen," "Knowing Me, Knowing You," "SOS" and "The Winner Takes It All."

As expected, Amanda says she had the jitters upon meeting the mighty Meryl, especially since she was playing her daughter in the movie, but the veteran Oscar winner "immediately took herself off that pedestal that she knew that I put her on. She's the most gracious, graceful woman."

And Jann was the first person to show Amanda her picture on the cover of the new Vanity Fair Young Hollywood issue, which calls her part of "The Next Wave" and pairs her with such up-and-coming actresses as Emma Roberts, Blake Lively and Kristen Stewart.

"God, I'm on it! This is the Vanity Fair cover and I'm on it! I can't believe it!" she exclaims. "Abba will last a long time in my mind ... I was the only one who felt that I was taken out by a gigantic wave."

Watch ET Monday night for more with the stars of 'Mamma Mia!'