Here is what we know about Giamatti's role (SPOILERS AHEAD): He will play the maverick, womanizing brother of Cora (Elizabeth McGovern), making him Mary (Michelle Dockery) and Edith's (Laura Carmichael) uncle. His character will appear in the Season 4 finale alongside returning guest star Shirley MacLaine.
Executive producer Gareth Neame doesn't elaborate on Giamatti's effect on the Granthams in the finale, but that he will, unsurprisingly, "upset their apple cart."
"Downton Abbey" Season 4 premieres stateside on PBS on January 5, 2014.
Giamatti won both an Emmy and a Golden Globe for his TV work in HBO's mini-series "John Adams," in which he played the title role.