Who's the daddy? Only ET's Jann Carl has the men of 'Mamma Mia!' -- Pierce Brosnan, Colin Firth and Stellan Skarsgard -- and they're telling all about the smash-musical-turned-cinematic summer sensation!

In theaters July 18, 'Mamma Mia!' follows an independent, single mother (Meryl Streep) and her spirited, about-to-be-married daughter (Amanda Seyfried) who is determined to find out the identity of her father so that he can walk her down the aisle. Pierce, Colin and Stellan's characters were all involved with Meryl's character 20 years earlier when they visited her Greek paradise, and any one of them could be the father!

When asked for a single word to describe co-star Meryl, Colin says, "Well, brilliant. She made it fun. She was absolutely approachable and very much in the spirit of it, and I think she's at the heart of why it works; she can be wonderfully kind of silly and abandoned one minute, and then she's breaking your heart."

"One word? Alive," offers Stellan, adding, "You don't have to be very smart to be an actor or actress, but Meryl actually proves that it helps."

And asked for a single word, Pierce can't contain himself: "Beautiful; magnificent; you have to use more than one word to describe her," he tells Jann. "When she sang, it remains etched in my mind. She was just so emotional, so powerful. You're an actor and an audience member at the same time."

As for that unscripted moment in which Pierce is soaking wet and dancing and Meryl rips off his shirt, he explains, "We were all dancing and enjoying the water, spring, life -- and she got very happy … [And I'm thinking,] should I do the same for her?"

For Meryl's take on the men of 'Mamma Mia!,' she tells Jann with a smile, "They were very game and willing to be sort of the secondary characters -- although I don't think they know that, so perhaps we shouldn't tell them; they still think the world revolves around them!"

Watch ET tonight for more with the men of 'Mamma Mia!'