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The actress will apparently play the small part of a wardrobe assistant named Lucy.
The film is based on the memoirs of Colin Clark and will also star Dame Judi Dench, Michelle Williams and Kenneth Branagh.
Clark's story documents the tense relationship between Laurence Olivier (Branagh) and Marilyn Monroe (Williams) during the production of The Prince and the Showgirl.
According to OK, Watson is able to shoot her scenes in just a few days to prevent her studies at Brown University being interrupted for a long period.
The film is being produced by David Parfitt and the Weinstein Company and will be directed by Simon Curtis.
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