She also had challenging choreography to master, twisting and twirling ropes with stripper-like moves. "The first day I saw the choreography...I said, 'You should almost fire me. I don't know if I'll ever get there.'"
But she did, and though her nerves got the best of her on the first day she recorded her song at London's famed Abbey Road recording studio, she produced a better recording -- and the one that ended up in the film -- on the second day.
Now Cruz said she would gladly do a musical again.
Cotillard, who won an Oscar for her performance as Edith Piaf in the French film "La Vie en Rose," always dreamed of being in a muscial. "Since I was young, I would watch 'Singing in the Rain' again and again, and I got pretty good at doing the choreography," she told Winfrey.
With the recent revival of the musical on the big screen, we are likely to see even more singing actors.
In Streep's case, she actually can sing. "She always had a reputation as being able to sing," New York casting director David Vaccari told ABCNews.com. "People have said that about Glenn Close, as well. People in the industry know they can sing. But not everybody in Middle America knows it."
Brosnan, on the other hand, can't sing, but in "Mama Mia!" it didn't seem to matter. Entertainment Weekly's reviewer wrote: "And just wait until Pierce Brosnan warbles 'S.O.S.' You'll laugh. And then you'll be charmed."
"In 'Mamma Mia!' you have a couple people gamely attempting to carry a tune," Carl DiOrio, deputy film editor at The Hollywood Reporter, told ABCNews.com. "Some pull it off better than others. But I haven't seen any evidence that Brosnan's lesser abilities in that regard will serve the movie in any sort of negative way. You almost have to give him points for being gamely enough in his efforts."
"Until they surface in vehicles like these, people not necessarily known for their singing abilities are going to rise and fall in whatever the particular vehicle they choose to sing in," DiOrio added.
Reese Witherspoon and Joaquin Phoenix rose to the challenge, doing their own singing in the Johnny Cash biopic "Walk The Line." Phoenix spent months at rock and roll boot camp, strengthening his voice and learning how to play guitar, to step into Cash's boots.
"I was surprised just to get through a song!" Phoenix told CNN in 2005. "I never sang before ... so to suddenly go to John was so odd to me. ... It was very strange to have to use a part of my voice that I didn't know existed."
Shocked by his Golden Globe win, he said, "Who would ever have thought that I would win in the comedy or musical category?"
Since quitting acting, Phoenix's new career as a rapper has been soundly dismissed.
But it was Witherspoon who took home the Oscar for her portrayal of Cash's wife, singer June Carter Cash. To prepare for the role, she took six months of extensive voice lessons and learned to play the autoharp.
Of course, she also had some singing chops to start with.
"I wanted to be a Broadway kid. I wanted to be on Broadway when I was 12, so I had singing lessons, but nothing prepared me for what it's like to stand in front of a microphone and hear it played back to you," Witherspoon told the Associated Press. "It's so humbling, and it made me really appreciate people who are naturally gifted."