For her role as Etta James in the upcoming drama Cadillac Records, Beyonce Knowles had to pack on 20 pounds.

See photos of Beyonce as Etta – and other celebs who played real people in movies.

Gaining weight “was so much fun,” she says in October’s British Marie Claire But when she later landed a part in the psychological thriller Obsessed, “I had to lose it,” she laments. “I was so angry with myself. I was like, ‘D’oh! Why do you have to go through this?’”

How strict is her diet these days?

Before her Marie Claire shoot, she stipulated her catering needs: “No chips, no candy, no desserts, no junk food, no fried food.”

See other stars who have undergone dramatic weight changes for roles.

The singer also opens up about her onstage alter ego, “Sasha,” adding that there are “things I do when performing I would never do normally.”

As “Sasha,” she says, “I have out-of-body experiences. If I cut my leg, if I fall, I don’t even feel it. I’m so fearless, I’m not aware of my face or my body.” READ MORE