<i>Mad Men</i> Mad Men

Cara Buono had no idea when she joined the cast of Mad Men's fourth season that she would still be hanging around for the season finale.

"I was told it was going to be a couple episodes, and it blossomed into this full story line," Buono tells TVGuide.com. "I wish I had a better word for amazing, because that's all I keep saying. But it really has been amazing."

Check out photos of the Mad Men cast

Buono plays Dr. Faye Miller, a consultant who came to Sterling Cooper Draper Pryce to do consumer research. She quickly caught the eye of newly single Don Draper (Jon Hamm

), but Faye didn't immediately hop into Don's bed like so many other women on the show."Faye is a certain kind of a woman we haven't seen before on the show," Buono says. "She and Don have a real relationship. Faye is not a mistress — she's his first real girlfriend. And it developed over time. They worked together and got to know each other. It developed slowly and quite unlike anything he's seen before."Still, the couple has at least one thing in common. "They're very good at their jobs and it's shaped who they are," Buono says. "We don't really know much about Faye's background. There's a mystery to her, in terms of what's gotten her there."On the other hand, Don's history as Dick Whitman has reared its head a couple times this season, and Don even told Faye the truth about his double life. Buono is clearly aware of the significance.

Read our Mad Men recaps

"That's a huge thing to reveal, and clearly he trusts her enough to be vulnerable with her," Buono says. "Like any good relationship, you are your authentic self in the relationship. Whether or not that is ultimately a good thing for Don Draper, I don't know."He is a completely self-made man who has been hiding his whole life — hiding who he is," Buono continues. "The season began with a question: 'Who is Don Draper?' Ultimately I wonder if the better question is: Who we are now versus who we were, and which one is more important to our existence in the world? ... I wonder if all of one's fears and secrets have to come out to really be who you are."But carrying Don's secrets could be quite a burden. In just the short time Faye has known Don, she's seen him become a sloppy drunk and have a full-on panic attack. Then, he used her to try to steal clients to save Sterling Cooper Draper Pryce when Lucky Strike, the agency's biggest client, took its business elsewhere.

A Mad dash to the finish

"I wonder what in her background makes her so strong and confident that none of this fazes her," Buono says with a laugh. "She seems to be able to take it in and she doesn't judge him and she doesn't run away. She's really standing by her man."But will she continue to do so? "Clearly there is chemistry and affection and great deal of respect in the relationship, but she's seen him at his darkest point," she says. "He kind of unloads a lot of baggage on her. That could create a very interesting dynamic between two people. We might want to talk again after the finale. Mad Men always surprises its audience."Mad Men airs Sunday at 10/9c on AMC.