The Irish American actress plays an English girl called Emma in the movie – which spans 20 years of her life – and she admits the book of the same name it is based on had too much character detail to include every aspect.
She told TV show 'BBC Breakfast': "We had to change some things about Emma, not huge things, but there were certain things about Emma that you get to know and there were certain layers to Emma in the book that were very difficult to make clear in the movie so we had to change it in order to communicate the story more effectively."
Anne – who stars alongside Jim Sturgess in the romance – also talked about the challenges of portraying Emma's ageing in the film, and how it is not as stark change as people might expect.
She added: "It was a definite challenge to figure out how to communicate aging without doing it dramatically, because people don't age dramatically, it's actually a slow thing then you wake up one day and you're older.
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"And you know you are and you don't know quite how you got there, so I thought if I do my job well people will arrive with Emma at the end of the film and not quite know when the changes happen but when you look back and she's at the end versus who she is at the beginning you'll see a definite growth."