The last time you saw Maggie Siff on television, she was fading, like a ghost, out of Don Draper's memory on one of the final episodes of Mad Men.

In a recent interview, Siff told me that scene — which called back to the events of the show's very first season, when she and Don enjoyed a brief but passionate affair with her character, Rachel Katz — almost featured a brand-new character: her unborn daughter. Mad Men creator Matthew Weiner invited Siff to shoot the scene just before the actress' baby was due, and he thought it would be a fun twist to show Don dreaming about a pregnant Rachel, and the life they could have had. But when Siff's day of shooting was delayed, her baby arrived in the real world, and Don's dream was necessarily changed.

That moment also neatly marked Siff's self-imposed work hiatus, as she settled into being a new mother. After working for years on the FX drama Sons of Anarchy and figuring prominently in Mad Men's bookend seasons, she felt entitled to choose her next project carefully.

"My husband always says to me, 'Saying no to one thing is saying yes to something else.' I tell myself that a lot in this profession," Siff says.

But to Siff's surprise, it wasn't long before a project she deemed worthy of a "yes" came calling. Siff is one of the stars of Billions, a new Showtime drama set in the cutthroat world of insider trading and federal prosecution in New York's financial district. Siff plays Wendy Rhoades — wife of one of the show's leads, U.S. Attorney Chuck Rhoades (Paul Giamatti), and therapist to the other, hedge fund billionaire Bobby Axelrod (Damien Lewis). As the two men circle each other in a cunning game of manipulation, Wendy is the only character on both sides, caught in the dissonance between her professional and personal duties.

When Billions came across her agent's desk — with Giamatti and Lewis already attached to star — Siff knew she wanted to audition, even if it meant cutting her break a little short.

"It was so smart and the character was so multi-dimensional on the page," she says. "You don't usually read roles where the female characters who are supporting characters are so multidimensional — especially in a pilot script."

Though Siff has played key roles in several prestigious shows, she said she rarely finds roles for women in male-dominated shows with the complexity and nuance of Wendy. "I've done my time playing women roles — interesting roles — where I am somebody's girlfriend or wife. As a woman, you get used to the feeling of being confined to a corner of the story," she says. "I was definitely excited at the prospect of, 'I get to be at home, and I get to be at the office, with a guy!'"

And as the link between the Rhoades and Axelrod portions of the show, Siff gets to act in two wildly different tonal environments. Playing an intimate relationship with Giamatti lets her unleash her inner passion and intensity, while Lewis' character keeps her on her professional toes, warm and supportive, but calculating all the time.

Despite the promising scripts and top-tier creative team, Siff did have some reservations about Billions. The first episode includes an S&M-style sexual dynamic between Wendy and Chuck that appears, on the surface, to be little more than premium-cable titillation for the audience. Siff has always prided herself on staying away from nudity for nudity's sake. But conversations with Giamatti and the showrunners reassured her that what was on the page would be rendered with sensitivity and nuance onscreen.

"I think people sometimes say, 'Oh, that's Showtime being naughty or trying to be risque.' I really think none of us wanted to feel like a cliche," she says. "There's certainly some humor in it. Our intention is not to shock and awe. Our intention is to add another interesting dimension to a relationship."

Roles like this don't come along often enough, Siff said, because there aren't enough women in positions of authority to write for them. She feels the tide starting to change, but not fast enough. Indeed, the four credited writers on Billions are all men. Still, Siff said she feels lucky that these men have taken care to craft female characters that are just as key to the show's overarching narrative. In fact — despite Billions' very strong cast — Siff ultimately makes the biggest impression.

Unfortunately, not every role offers so much range. Siff tells me she's only in the background in a major motion picture out later this month: the YA sci-fi adaptation The Fifth Wave, starring Chloe Grace Moretz. "Blink and you'll miss me," she says with a laugh.

Indeed, major studio films have largely eluded Siff thus far. But she's still interested in taking her talents to the big screen, and the brisk shooting schedule for a 13-episode cable drama still leaves plenty of room for side projects. With any luck, Billions — which provides as strong a showcase as she has ever had — will remind everyone what she has to offer.