George was pictured alongside Phyllis Logan (Mrs Hughes), Laura Carmichael (Lady Edith), Lily James (Lady Rose), Michelle Dockery (Lady Mary) and Elizabeth McGovern (Lady Cora) while filming his cameo on the ITV show – and understandably there were big smiles on set.
George Clooney with the delighted ladies of Downton Abbey
Elizabeth and Lily also appeared with George in a teaser trailer for his special appearance, which is due to air on 19 December.
In the clip, Cora can be seen discussing a dinner invitation to the back of someone viewers are led to believe is her husband, The Earl of Grantham (Hugh Bonneville). But, as the song Let It Snow plays in the background, the man turns around revealing himself to be George, looking dapper in a three-piece suit.
George, 53, is thought to have taken on the role in the one-off sketch, for ITV's Text Santa campaign, after striking up a friendship with Hugh on the set of their film Monuments Men.
Details of his character have remained shrouded in secrecy, but that hasn't stopped some cast members speaking about their experiences filming alongside George.
George Clooney pictured with Hugh Bonneville
"He came and filmed with us for an afternoon back in May when we were doing this Text Santa sketch and he was incredibly generous with his time and his talent," Hugh said.
"It's interesting the number of people who suddenly came on set who'd never been on set in five years of filming! He was great – he was a good laugh and sprinkled some sort of magic dust over our experience."
Jim Carter, meanwhile, who plays butler Mr Carson on the show, joked that he had offered George some advice on smartening up before shooting began.
"He asked me for a few tips actually. And I said, 'George, George, George, just try and spruce yourself up because you're a sadly plain creature'," he joked. "He was thankful for my advice."