Aside from plugging her much-awaited motion picture, Miss Mara talked with Jimmy about sitting on Santa's lap during her family's Christmas Eve get-together.
Of the family tradition, Rooney dished, "There's over forty grandchildren and every Christmas Eve we go to their house [her grandparents] and they have Santa Claus come and we have to sit on his lap to get our present."
Curious as to who takes on the role of St. Nick, Fallon asked, "Is it just a stranger, like a drifter off the street or do you hire a real Santa Claus?"
"No, it's usually like one of my uncle's brothers or something," Mara replied before Jimmy chimed in, "So I guess it would be your uncle..."
Clarifying things up, Rooney said, "No, no. Not blood related," to which Fallon joked, "Oh, not blood-related? That's a rule [laughs]. You can't be blood-related or you can't play Santa!"