"Initially the directors saw my rehearsal with Destiny's Child, and I was the over [the] top strong belting Sasha, and they were, I think, a little shocked. 'Well, maybe she … maybe she isn't Deena,'" Knowles said.
"They were concerned that young Deena -- who was very shy and naive -- they were afraid that people wouldn't be able to look at me as that character."
Nevertheless, Knowles got the part. And you see the transformation on film -- bold Beyonce becomes meek Deena.
"When I watch the movie, I don't see myself at all," Knowles said. "There are certain moments when I watch the movie and I'm so upset. … At one point that I had to look at my mom, and say, 'Oh I can't stand Deena.' Like it wasn't me. But that's how I know I did my job."
That very weakness and timidity, however, provide a point of contrast for the transformation that Deena finally undergoes.
"By the end, I have my strength. By the end, I find my voice," Knowles said. "And it's because she is so passive and she completely has no opinions, when she finally does it's so effective and so powerful."
This struggle between competing emotions provided fodder for Knowles' new CD, "B'Day," which she said she recorded over just a couple of weeks, jumping between three recording studios within days of finishing "Dreamgirls."
"I am not sure how actors release a character if they don't have an album or music. I guess they just go and make another movie and get into another role," Knowles said.
"I used making this record as a vehicle to find myself again, so I let out a lot of emotions that Deena was feeling that I wasn't, and I am so happy because I feel like I was able to connect with people that are going through certain things that I am not."
Some of Deena's emotions seem pretty darn angry. Knowles herself may not be angry, but she is restless, and also disciplined.
To capture the look she wanted for "Dreamgirls," she shed 20 pounds.
"Diana Ross was my inspiration as far as the look, and she was a lot thinner, and I wanted to have that look so I went for it," Knowles said. "No one asked me to do that. That's just something that I thought would be good for the character."
To slim down, Beyonce kept herself to a "cleanse" diet that involved no solid food, just cayenne pepper, maple syrup, water and lemon.
"It was very spicy, and I was grouchy and hungry for a long time," Knowles said. "It was difficult. I'm a Southern girl. I grew up with huge portions of food, with candied yams and macaroni and cheese. Everything fried."
Knowles is now back to what she calls "Beyonce" weight. And now that she believes in herself as an actress, she has characteristically big goals.
"I want to be the first African-American woman to have a Tony, an Oscar and Grammys," Knowles said.
But she is modest about "Dreamgirls."
Knowles admits this may not be the role that snags her a golden statue, though she hopes it will prove to her detractors that she is a serious actress.
Oscar or no, Knowles confesses the role may well become part of her legacy.
She admits it has crossed her mind that someday another young actress might play the 25-year-old diva at the height of her talent and celebrity.
"When I think about Diana Ross playing Billie Holiday and now I'm playing someone loosely based off of Diana Ross, maybe in 25 years, someone will be playing me," Knowles said. "And that's so amazing and beautiful.