Minka Kelly listened to Jackie Kennedy tapes while sleeping

Minka Kelly was so determined to master Jackie Kennedy's speaking voice in The Butler, she went to sleep listening to tapes of the former First Lady.

The actress tackled the role of President John F. Kennedy's wife for Lee Daniels' new film, and admits it was a challenge to master the fashion icon's distinct accent.

She tells People.com, "I think she spoke in a way she thought she should speak, so getting that down was hard. There's a musicality and rhythm to the way she speaks. I went to sleep listening to her."

Kelly also struggled with Kennedy's retro outfits, adding, "My body is so different from her because I have curves, so fitting into those vintage clothes was actually really hard. Also it was hot - and there was a lot of wool!"

The movie chronicles the life and career of Eugene Allen, who served as the White House butler for 50 years.

The all-star cast includes Forest Whitaker as Allen and Oprah Winfrey as his wife, while Jane Fonda portrays another First Lady, Nancy Reagan.