The 31-year-old British actress, who plays Margaery Tyrell on the HBO series, will portray Cressida, a director from the Capitol who joins Katniss in the uprising.
Mockingjay, the third book in the "Hunger Games" franchise, will be divided into two movies, the first of which is set to debut on Nov. 21, 2014 with the second installment scheduled for Nov. 20, 2015.
The second film of the saga, "Catching Fire" hits theaters Nov. 22.
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