"They’re very good at keeping things top secret on the show," Campbell told EW. "When I first got to set and was in the hair-and-makeup trailer, someone said, 'Oh, [showrunner] Matt Weiner’s coming down to talk to you,' and the hair-and-makeup people said, 'Oh, he’s coming to give you the super-secrecy speech.'"
"He came down and gave me a big speech about the importance of keeping things hush on the show, and keeping things entertaining and fresh for the audience, which I completely respect, and how much they need people to not say a word," she added.
Showrunner Matt Weiner likes to work surprise guest stars into Mad Men episodes. Gilmore Girls' Alexis Bledel turned up in season five, while Freaks and Geeks star Linda Cardellini, who was also asked to keep quiet, made her debut in season six.
Jon Hamm, who plays Don, told the Daily Beast that his intimate conversation with Campbell's character represents a new direction for his character.
"I thought it was a beautiful, weird opening up for Don -- to have this conversation with a stranger. It was remarkably affecting. I was like, 'Whoa,'" he said.
"I’d forgotten about it," he added. "But it’s a beautiful moment, and she’s so good in it. It’s nice to see her again on screen. It’s this weirdly tender moment that kind of comes out of left field. I was pleasantly surprised by it."
Celebrity Families of 2014 [PHOTOS]