Oscar-nominated actor Paul Giamatti will be joining the cast of Downton Abbey for the Christmas Day special.
The American star, who was nominated for the best supporting actor Oscar for his role in boxing drama Cinderella Man, will play Lady Grantham's brother.
Not a lot has been given away about Giamatti's role in the ITV drama, but producers have let slip the Christmas Day episode will revolve around Lady Rose MacClare's (Lily James) entry into London's high society.
Shirley MacLaine returns as Lady Grantham's mother Martha and James Fox joins the cast as aristocrat Lord Aysgarth.
The two-hour episode of the award-winning ITV series, which will be shown at 8.30pm on Christmas Day, picks up the story six months on from the end of series four.
The Prime Minister's brother, Alexander Cameron QC, made an appearance in some of the first footage to be broadcast from the Court of Appeal since 1925.
Television pictures of a live case at the Court of Appeal have been broadcast for the first time since cameras were banned from the vast majority of courtrooms in 1925.
The landmark footage did not include any sound since the case in question involved sensitive details relating to a child.
The defendant was not in shot since the cameras in the court will focus on the lawyers' arguments and judges' summing up, decision and - in criminal cases - sentencing remarks.
A near-90-year ban on filming in court will be lifted today in what has been hailed as a "landmark moment for justice and journalism".
For the first time, cameras will be able to broadcast from one of the highest courts in the land, the Court of Appeal.
After years of campaigning by broadcasters BBC, ITN, Press Association and Sky News, cameras have been placed in five courtrooms at the Royal Courts of Justice.
From tomorrow cameras will be allowed to film at the Court of Appeal. After 90 years, the ban on filming proceedings there has been lifted. We will be able to see the legal arguments and the judge's ruling. But the defendants and witnesses will not be filmed.
And it could pave the way for cameras in other Courts.
ITV News' Paul Davies reports:
The legal system in England and Wales has reached a landmark for open justice: from tomorrow, broadcasters can film in the Court of Appeal.Read the full story ›
Footage film in the court room can be used in a news and current affairs context only and is banned from being used in other genres such as satire, entertainment or commercial use in advertising.
ITN chief executive John Hardie said: "Filming in courts has been a long time coming and is for the benefit of open justice and democracy. Never before will television viewers have had such an insight to justice being seen to be done."
Camera positions are to be operated by the court video journalist with both legal and journalistic qualifications.
Safeguards have been put in place to protect witnesses, victims and the administration of justice while ensuring cameras in the courtroom do not disrupt proceedings.
Some cases will be broadcast live with a 70-second delay to allow the removal of anything that contravenes broadcasting regulations or standard court reporting restrictions - such as contempt of court laws and court orders.
In addition, appeals against conviction which might result in a re-trial will only be shown once the case is decided, and the judge can order no filming or broadcasting if it is in the interests of justice.
The top judge in England and Wales has said that allowing cameras in courtrooms will allow a "wider audience" to understand and see for themselves the workings of the legal process.
Lord Chief Justice Lord Thomas, said: "My fellow judges and I welcome the start of broadcasting from the Court of Appeal.
"The Court of Appeal has, of course, been open to the public and to journalists for a long time.
"The change in the law which is now coming into force will permit the recording and broadcasting of the proceedings of the Court of Appeal.
"This will help a wider audience to understand and see for themselves how the Court of Appeal goes about its work."
Victims, witnesses and defendants will not be filmed while cameras instead focus on lawyers' arguments and judges' summing up, decision and - in criminal cases - sentencing remarks. Proceedings will be filmed from only one courtroom on any given day.
The lifting of the ban means it will be the first time that cameras have been allowed in courts other than the Supreme Court since filming was banned by the Criminal Justice Act 1925.