The actress - who plays housemaid Anna Bates - discussed working with the two iconic stars in a new interview with People.
"This scene I was in, there's an amazing little conversation between the two of them, a sort of snipey conversation," Froggatt hinted.
She continued: "It was like watching these two legends at work. It was one of the most special days I think I've ever had on set. It was magical."
Froggatt also offered a preview of her character Anna's struggle to prove husband John Bates (Brendan Coyle) is innocent of murder charges.
"You do see her grow up that little bit more, and she's a married lady now, she's Mr Bates. She moves into a more senior position in the house and she kind of becomes a woman," the star revealed.
"Obviously her and Mr Bates are having their turmoils again, so she's fighting to free her man as well."
Earlier this week, the first footage of MacLaine as Martha Levinson in Downton Abbey was unveiled.
Downton Abbey will return to ITV in the UK in late 2012. In the US, the series airs on PBS.
Watch the Downton Abbey cast discuss series three below: