Actress Linda Cardellini had to arrive on the Mad Men Tv set incognito, so even her new co-stars wouldn't know who the mystery newcomer was or what role she'd be playing.
The new mum returned to acting after giving birth to her daughter Lilah-Rose last year (12) by agreeing to play Mad Men hunk Don Draper's neighbourly mistress, who made her debut in the show's latest season premiere when it aired in America on Sunday (07Apr13).
And she admits show creator Matt Weiner was so determined to keep the Scooby-Doo and Er star's casting a secret he instructed her not to tell anyone and arrive on set in disguise.
Cardellini tells Entertainment Weekly, "No one knew about it, even my friends and family. When people figured out I was working, I just said it was an independent film and nobody had further questions.
"Matt asked me to wear a hat to set, and there were times where I'd be in a van with the windows covered or someone would take me to set with an umbrella. Matt was really excited about it being a surprise that I'm on the show."
Now that her character, Sylvia, is no longer a secret, the actress, 37, is bracing herself for a backlash from fans of the Tv drama, who hate the idea of a new woman coming between Jon Hamm's Draper and his wife Megan, played by Jessica Pare.
She adds, "I think I'll have some harsh words coming my way."