'The New York Times'To think there was a time that a television character played by Linda Cardellini was regarded as a freak simply because she dropped out of her high school Mathletes club.

But that was a different decade, on a different period drama, and ever since this actress showed up on Mad Men , she has brought a whole new calculus to that AMC series and to the life of Don Draper.

Cardellini, who played the melancholy teenager Lindsay Weir on Freaks and Geeks (which ended in 2000), made her Mad Men debut this season, playing Sylvia Rosen, a seemingly demure housewife and Don's mistress . Their assignations took many twists and turns - and got downright kinky - before Sylvia called it off . For Cardellini, the role of Sylvia Rosen has been a secret she's had to keep from friends and colleagues. In an interview she spoke about her 1960s alter ego and the element of surprise. Here are excerpts from that interview.

___ Has the gag order been lifted? Are you allowed to go outside your house?

___ The house arrest is lightening up [laughs]. Nobody really knew that I was doing this. It's rare that you can keep that kind of secret, especially from people within the industry. It's been fun to see friends who I've acted with over the years find out that I'm on the show.

___ How much information are you given about the character when you first come in to read?

___ You know nothing about what it is you're doing when you come in. The character's name was Sylvia, but I did not know it was Don [whom she was interacting with] at that point. I've never worked in that way, and it kept me in the moment. You can't play the end of your arc , because you don't know it yet.

___ Did Matt Weiner, the creator of Mad Men share other details about Sylvia once you'd landed the role ?

___ Matt and I had long talks about it, and he revealed certain things to me. But Don's arc is so deeply structured - Matt always has a master plan, but the details are being worked out . So Matt would tell me things about the character, things that weren't on the page, but I always wondered if my curiosity was driving him crazy , because every time I saw the guy, I was like, "What else can you tell me?"

___ What kinds of things did you talk about early on?

___ From the beginning, it was always very important that she's always wearing her crucifix, and something that you don't even come to see until Don mentions it. She's so conflicted over these things. She's so assured about her faith and yet she's doing something that's very sinful .

___ Yet you couldn't really tell people that you were playing this character, or even that you were on Mad Men .

___ No. I couldn't even tell people I was working [laughs]. But after having a new baby, people weren't really asking that much, anyway.

___With Mad Men and Freaks and Geeks , you're now associated with two television shows that take place in past decades. Do you see differences in how they approach the eras they're set in?

___They seem drastically different in terms of how the period is played. With Freaks and Geeks , we were aware we were in 1980, we were wearing different clothes, and our hair was different, but we were just playing teenagers. I don't think being a teenager in 1980 made a difference to me in terms of how I thought of being a teenager at the time. There wasn't the cultural upheaval and the backdrop . [On Mad Men ] the culture and the mores of the 1960s play heavily into the characters. What everybody's going through is timeless or it wouldn't translate.

___I'd say Sylvia is about as far as you can get from one of your other roles, Velma in the movie Scooby-Doo .

___ Yes, completely. Really, what my character does is spend a lot of time in the bedroom with Don. Even my mother said, "Are you ever going to get out of the bed?"

___Yes, you got onto the floor.

___I did. I'm sure she was happy about that. But I've played girls for a very long time, and it's nice to play a woman with her own sense of sensuality. To be the woman who can bring Don Draper to his knees is fun.