According to Tom Cullen, who plays Lord Gillingham in the ITV period drama, 79-year-old Dame Maggie "collapsed onto the sofa" after accepting George's affectionate offer.
Tom Cullen told Lorraine Kelly about George's visit to the Downton set
Speaking to presenter Lorraine Kelly, Tom described George's time on the Downton set.
"I know he gave Maggie a kiss and she collapsed onto the sofa..." he said. "He’s a very handsome man, isn’t he?"
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While the 29-year-old actor was not on location at Highclere Castle on the same day as George, he revealed that the Ocean's Eleven star shares screen time with Michelle Dockery and Hugh Bonneville.
It is 50-year-old Hugh who is credited with asking George to appear in the charity sketch, which will air at Christmas, after they worked on The Monuments Men together.
George Clooney swept Dame Maggie Smith off her feet with a kiss
"All the plans have been shrouded in secrecy because George was determined for his role not to leak out," a source told the Sun of George's high-profile guest appearance. "All the other cast members and crew were sworn to secrecy.
"It's amazing it's taken four months for someone to find out," they added. "This is the biggest moment in Downton history and shows it's now the biggest drama in the world."