The Pitch Perfect star, 29, admitted to ET that she originally wanted to play Little Red Riding Hood.
"They told me an actual 12-year-old [Lilla Crawford] would play Little Red Riding Hood," she told ET at the Into the Woods New York premiere on Monday. "In the show it's a grown woman, so I was like, 'Put me in! I can do it!'"
Based on the Broadway musical, Into the Woods follows the classic tales of Cinderella, Little Red Riding Hood, Jack and the Beanstalk and Rapunzel, when a baker (James Corden) and his wife (Emily Blunt) want to begin a family despite their interaction with a witch (Meryl Streep), who put a curse on them.
"This was a dream come true," said Anna. "When I found out that they were making this into a movie, I just hoped and prayed and wished and wished, and it's such a gift. I feel forever grateful to [director] Rob Marshall."
While she couldn't have asked for a better project, the role didn't come without difficulty. Anna told us that she had one of the toughest songs in the movie.
"It's a register I haven't sung in before," said Anna. "I hope I'm not being biased to say I think it's one of the most challenging songs in the piece."
As if her Into the Woods songs weren't challenging enough, Anna also stars in another musical, The Last Five Years, where she really has to sing her heart out.
Into the Woods opens Christmas Day.