The Golden Globe winner will appear in the Christmas special as a 'maverick playboy uncle'

Sideways star Paul Giamatti is to join the cast of Downton Abbey.

The Golden Globe-winning actor will appear in the show’s Christmas episode as Harold, the “maverick playboy brother” of Elizabeth McGovern’s Cora. Shirley MacLaine, who guested on the hit costume drama last year (2012), will reprise her role as Martha, Harold’s mother.

“We’re excited that Paul Giamatti will be joining us on Downton to play Cora’s brother Harold, the rather free-spirited uncle to Mary and Edith,” said Gareth Neame of Carnival Films, the makers of Downton Abbey. ” We can’t wait to see him work alongside Shirley MacLaine, who are both sure to upset the Granthams’ apple cart in this year’s Christmas Day episode.”

The Christmas Day episode will mark the climax of Downton Abbey‘s fourth season, which begins on ITV1 this autumn. Maggie Smith, Hugh Bonneville and Michelle Dockery will all be back as members of the aristocratic Grantham family.

Created and written by Julian Fellowes (pictured above right with Bonneville and McGovern), Downton Abbey premiered on ITV1 in 2010 and has gone on to become a major international smash. Over 120 million viewers worldwide are now estimated to have watched the series.

Meanwhile, Giamatti is set to play a supervillain called The Rhino in The Amazing Spider-Man 2, which is shooting now in New York. He’s best known for starring in the acclaimed indie comedies Sideways and Barney’s Version as well as his supporting roles in Saving Private Ryan, The Ides Of March and The Truman Show.