AceShowbiz - Actress Maggie Siff was initially shocked to discover her therapist character on TV drama Billions was secretly into sadomasochism, because she had fallen asleep before reading the full script.

On the show, the Sons of Anarchy star portrays Wendy Rhoades, a tough-talking counsellor by day who enjoys spending her nights getting kinky with her husband, U.S. Attorney Charles 'Chuck' Rhoades, Jr., played by Paul Giamatti, but Siff confesses she wasn't fully aware of her character's dominatrix desires when she first became taken by the role.

"I read about three quarters of the script, and then I went to sleep, and I was like, 'This role is incredible!,'" she recalled on breakfast show Today. "And then I woke up the next morning and I finished it, and I was like, 'Oh, wait a minute!' She does some naughty things!"

The unexpected personality traits made Siff rethink her approach to the onscreen persona, and the more she thought about it, the more she fell in love with the idea of leading a secret sex lifestyle.

"I kinda had to recalibrate my understanding of who the character was, but it made it kind of actually more interesting," Maggie said. "The thing that I love about the character is how many different facets and dimensions you get to see her inhabit and that (her interest in S&M) is a really significant one. It shocks an audience and it also reverberates and ripples through the different parts of her life in really interesting ways."

The role of Wendy is one Maggie has now played for three seasons to date, and working alongside Giamatti and Damian Lewis has proved to be one of Siff's personal career highlights.

"Damian is so scintillating and witty and incredibly funny, and he'll burst into song, and he kinda like, tickles you and delights you with his charm and his wit, and he's very British...," she smiled, before adding of her onscreen husband Giamatti: "Paul is just... hilarious. He is so funny, he has the best sense of humour."