The 79-year-old actress, who plays the Countess of Grantham in the period drama, will swoon as she receives a kiss from Clooney in the episode, according to her co-star, reported Telegraph online.
The usually-dignified Countess is expected to faint in a "comedy fall" on-screen, as Clooney joins the regular cast for a cameo role in a charity episode of 'Downton'.
"There is a very brilliant moment with Maggie and George - and I don't know if I want to give away any more than that really - but Maggie does a very comedy fall," Laura Carmichael, who plays Lady Edith in the show, said.
Clooney, 53, secured a part on the show after becoming friends with actor Hugh Bonneville.
His appearance was confirmed at a premiere of the first episode of the new series of 'Downton' by writer Lord Fellowes.