MacLaine was added to the cast for Season 3 of the ITV drama -- set in the early 20th century -- which airs in the United States Sundays on PBS under the network's Masterpiece programming banner. The actress plays Martha Levinson, the mother of Elizabeth McGovern's character Lady Grantham.
MacLaine's character is meant to provide a contrast to Smith's prim, upper-crust traditionalist Lady Violet, mother of Lord Grantham.
"[Levinson is] a force for change. She goes to see the other side of her family and talks about how they got into a world war because they were addicted to tradition. She tells them they really need to get up to speed on what's happening in the world," MacLaine said in an interview with Parade Magazine.
In one scene, her character sings "Let Me Call You Sweetheart" to Lady Violet at a party.
"That was my idea, and Julian [Fellowes, 'Downton' creator-writer] went for it. I thought it might be funny and charming -- how would this character who doesn't even know what a weekend is react to someone singing a love song to her?"