The 69 year old admits she showed her bare bottom to Warren Beatty while they were working together on 1981 movie Reds in a bid to make him crack a smile.
She tells U.K. talk show host Graham Norton, "He is grumpy. He really is. So I mooned at him."
Margolyes also reveals she stripped off in front of director Martin Scorsese on the set of The Age of Innocence, adding, "I didn't moon him, I did these instead (my breasts).
"It was a long day and we'd done the costume parade at the beginning (of the film). (Everyone was) feeling tired and a bit listless, so I just flung off my brassiere."
But the actress was banned from bringing her naughty antics to the set of the Harry Potter movies, and instead her colleagues set up a 'swear box' which she had to contribute to every time she cursed in front of her young co-stars.
She adds, "You have to be very careful with children and they made a rule that I had to give a penny or maybe a pound to the World Wildlife Fund if I swore and, really, they did very well."