MacLaine will reprise her role as Martha Levinson, Robert Crawley's freewheeling American mother-in-law, Carnival Films and "Masterpiece" on PBS said Saturday. MacLaine appeared in episodes early last season.
New Zealand-born soprano Te Kanawa will play a house guest. She will sing during her visit.
Other new cast members and characters include:
• Tom Cullen as Lord Gillingham, described as an old family friend of the Crawleys who visits the family as a guest for a house party (and who might be the one to mend Lady Mary Crawley's broken heart).
• Nigel Harman will play a valet named Green.
• Harriet Walter plays Lady Shackleton, an old friend of the Dowager Countess.
• Joanna David will play a guest role as the Duchess of Yeovil.
• Julian Ovenden is cast as aristocrat Charles Blake.
"The addition of these characters can only mean more delicious drama, which is what 'Downton Abbey' is all about," said "Masterpiece" executive producer Rebecca Eaton.
Meanwhile, the producers have confirmed that villainous housemaid Sarah O'Brien won't be back. Siobhan Finneran, who played her, is leaving the show.
These announcements come shortly after the third season's airing in the United States. It concluded with the heartbreaking death of popular Matthew Crawley in a car crash, leaving behind his newborn child and loving wife, Lady Mary Crawley.
Matthew's untimely demise was the result of the departure from the series by actor Dan Stevens, who had starred in that role.
The third season also saw the shocking death of Lady Sybil Branson, who died during childbirth. She was played by the departing Jessica Brown Findlay.
Last season the wildly popular melodrama, set in early 20th century Britain, was the most-watched series on PBS since Ken Burns' epic "The Civil War," which first aired in 1990. The Nielsen Co. said 8.2 million viewers saw the "Downton" season conclusion.
"Downton Abbey," which airs on the "Masterpiece" anthology, won three Emmy awards last fall, including a best supporting actress trophy for Maggie Smith (the Dowager Countess), who also won a Golden Globe in January.
In all, the series has won nine Emmys, two Golden Globes and a Screen Actors Guild Award for the ensemble cast, which is the first time the cast of a British television show has won this award.
Hugh Bonneville, Michelle Dockery, Elizabeth McGovern, Jim Carter and Brendan Coyle are among its other returning stars.
Read more: www.pbs.org/downton
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