The actress, who plays Emma in the adaptation of David Nicholls's 2009 novel, said that her accent is "designed to change" since the character spend her early years moving around.
"Emma is from Leeds but has been at university, and then goes to London, so the accent is designed to change the way it does when you leave home, particularly in the UK," Hathaway told Grazia.
"I met ten people from Yorkshire and nine of them didn't sound like they were from Yorkshire anymore."
The Dark Knight Rises actress recently revealed that she has changed her views on marriage after watching a friend walk down the aisle, confessing that she is now open to the institution after seeing how happy it can make people.
Hathaway previously claimed that she relates to her One Day character in that she too feels insecure at times.
"Confidence is not something that blooms overnight," she said. "It takes time to develop - at least it has for me."
Watch the trailer for One Day below: