04/07/2013 11:06 pm ET Updated 4 days ago

Linda Cardellini On 'Mad Men': 'Freaks And Geeks' Star Portraying Don's Mistress (VIDEO)

Note: Do not read on if you have not yet seen the Season 6 premiere of AMC's "Mad Men," titled "The Doorway."

On Sunday's Season 6 premiere of "Mad Men," viewers may have been surprised to see a familiar face of the '90s in the Draper apartment: "Freaks And Geeks" star Linda Cardellini.

Cardellini played Sylvia, a woman who lives in Don (Jon Hamm) and Megan's (Jessica Pare) building, and also happens to be Don's latest mistress. Can't say we didn't see that one coming, but we certainly didn't expect Cardellini to be the new "other woman."

"Usually people know what you’re doing, especially with the Internet. People know what you’re doing sometimes even before you know," the actress told Entertainment Weekly of keeping her "Mad Men" role a secret. "To have it be a surprise for my friends and family who haven’t seen me do anything for a while because I’ve been home with my daughter, it’ll be a really fun surprise.”

The "Mad Men" cast isn't completely new to Cardellini. "I'd done a movie before with Vincent Kartheiser and I had done another movie recently with John Slattery, so I had acted with them before," she told EW. "It’s a very, very good group of people that have been good to me. I wish I could tell you more stories, but then it would allude to who I hang out with on set. It would give away certain things that I probably can’t."

Cardellini's "Mad Men" appearance marks the third teen star to join the AMC series. "Gilmore Girls" alum Alexis Bledel played Pete Campbell's (Vincent Kartheiser) mistress last season, while Larisa Oleynik of "Alex Mack" fame portrays Ken Cosgrove's (Aaron Staton) wife Cynthia.

In case you want to relive Cardellini's "Freaks And Geeks" glory as Lindsay Weir, check out the video below:

For more from Cardellini on "Mad Men," click over to EW.

"Mad Men" airs on Sundays at 10 p.m. ET on AMC.