Unlike a lot of Hollywood actresses, Meryl knows when to have fun with her films. Nobody thought (starting with her, I'm sure) that she would be Oscar-nominated for what was released as a summer popcorn chick flick in 2006. What I love the most about seeing Meryl play this comic characters is that you can tell she is having so much fun on screen, that's why I love SHE-DEVIL and DEATH BECOMES HER.
From the MAMMA MIA trailers, you can tell she is just having a good time. She's got nothing to "accomplish" by making this movie. She's American royalty. And it is probably her versatility as an actress and her ability to take all kinds of roles (this year she'll be starring in MAMMA MIA and DOUBT, complete opposites) that has kept her getting roles again and again at an age when a lot of other actresses are struggling to stay in the business.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"