CHICAGO - Marion Cotillard says the most challenging part of playing John Dillinger's love interest in the upcoming film "Public Enemies" was developing a believable Midwestern accent.

The Academy Award-winning actress worked for four months with a dialect coach, two hours a day, to tone down her natural French accent.

"It was really weird because she taught me how to use my jaw and my tongue in a different way," she said during a recent interview.

Her biggest challenge?

The R and the L.

"I think it was the hardest thing that I've ever had to do," she said.

Eventually, Cotillard — who won a best-actress Oscar for playing chanteuse Edith Piaf in "La Vie en Rose" — pulled it off to play her half-French, half-Indian character, Billie Frechette.

Michael Mann, the director of "Public Enemies," said he had seen Cotillard's previous movies and knew she could overcome speech problems.

"I knew mechanically, with fantastic discipline, a great work ethic and just hard work, she will master the English," he said.

After "Public Enemies," which stars Johnny Depp and Christian Bale, Cotillard played a character who had a French accent over a British accent.

Her biggest problem: the R's and the L's — again.

"I was like, `Are you kidding me?! It took me four months to get this R and L,'" she said.

Accent has been a constant challenge for the 33-year-old Cotillard, who acts in French and English movies. She learned English when she was 11. When she got her first English role (in Tim Burton's "Big Fish") she went to the Berlitz language center to work on her English accent.

Then, she returned to France for a role in her first language. On the first day, in the first scene, she spent three hours on the very first line.

Despite the constant stream of dialect coaches, Cotillard said she likes the challenge of changing her accent for roles. She said it makes her appreciate success more.

"I like when it's really something that you don't get right away," she said. "You just have to work and work and work."