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Downton Abbey Exclusive: Paul Giamatti Joins Season 4 Cast as Cora's [Spoiler]

Downton Abbey Casts Paul GiamattiDownton Abbey is bracing for another American invasion.

TVLine has learned exclusively that Paul Giamatti (Sideways, Cinderella Man) is joining the Masterpiece/PBS phenom’s Season 4 cast as Harold Levinson, the maverick, playboy brother of Cora (Elizabeth McGovern) and uncle to Mary (Michelle Dockery) and Edith (Laura Carmichael).

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Cora-abbeyGiamatti, who earned an Emmy and a Golden Globe for his title role in HBO’s miniseries John Adams, is set to appear in the Season 4 finale alongside returning guest star Shirley MacLaine (as Cora and Harold’s mother, Martha).

Exec producer and Carnival Films Managing Director Gareth Neame describes Harold as “free-spirited” and promises that both Giamatti and MacLaine will “upset the Grantham’s apple cart” in the finale.

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Giamatti joins fellow Season 4 newbies Dame Harriet Walters (as an old friend of Maggie Smith’s Dowager), Nigel Harman (as a visiting valet named Green), and Tom Cullen (as a new love interest for Mary).

Abbey‘s fourth season will premiere stateside on PBS on Jan. 5, 2014.