The Josie Rourke-directed film will center on the rivalry between Queen Elizabeth (Robbie) and her cousin Mary (Ronan). Alwyn will play Dudley, Queen Elizabeth’s counselor and lover.
Mary Queen of Scots is based on John Guy’s 2004 biography, The True Life of Mary Stuart.
Since starring in his first major film role in Billy Lynn’s Long Halftime Walk, Alwyn has been cast in another major film, The Favourite, alongside Emma Stone.
Earlier this month, Swift, 27, and Alwyn were spotted together for the first time after secretly dating for months. The singer and the actor are longtime friends, and Swift was first spotted publicly with Alwyn last fall after attending a Kings of Leon concert with a group of her pals.
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In addition, Alwyn has already met her parents. He jetted to the States from the U.K. with his girlfriend to spend some quality time with her family in Nashville.