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While there are plenty of racy reasons to tune into Masters of Sex, it's the immaculately styled '50s and '60s-era costumes that add that something extra. The Showtime series, which is back for Season 3 this Sunday at 10 p.m. ET, kicks off in 1965 with sex researchers Dr. William Masters (Michael Sheen) and Virginia Johnson (Lizzy Caplan) gearing up to publish their book, The Human Sexual Response.

And for a show that is essentially about taking clothes off, it's sometimes surprising to see sophisticated outfits make such an impact. That contradiction is not lost on the show's new costume designer, Isis Mussenden. "I was intrigued that you could do a show about the sexual revolution in a way that's not overt when it comes to the clothing," Mussenden says. "It’s never about cleavage and body parts showing. Yes, the outfits our actors wear are good-looking, but it is what's simmering underneath the clothes that makes the show so hot."

We caught up with Mussenden to get the scoop on her retro inspirations and the style that we can expect to see this season.

The third season starts off with a big time jump to the mid-'60s. How did that affect what the characters will be wearing?

The look is different, especially because it was a transitional period for women. It was a time when suddenly the hemline was changing. I could trace the first miniskirt back to a Paris runway in 1964, but it still took a few years for it to trickle down to a woman, in her thirties, living in St. Louis. There are also a lot of new characters that will be coming into the institute—from swingers to hippies—that will allow us to touch on other aspects of the time period.

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In the first episode, we see Virginia in a tailored skirt suit and Dr. Masters in a suit and bow tie for the press conference where they debut their book. What was the inspiration behind these looks?

For Virginia, we mimicked the shell, skirt, and jacket from an outfit that the real Virginia Johnson wore to a press conference. With her, we also often draw inspiration from pictures of Natalie Wood and Audrey Hepburn. Her clothes have to be conservative because she wouldn’t have been taken seriously in her career otherwise. This is an era when it was unheard of for women to wear pants to work. As for Masters, he was a man of habit and he didn’t change his look for 30 years. His shirts and suits are all custom-made, as are most of his ties. For Michael, that’s why it all fits so well!

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How does the style of Libby Masters (Caitlin FitzGerald) differ from Virginia?

Libby is Masters’s wife, who has always been fashionable and beautiful. She’s kind of our Vanessa Redgrave, mixed with a little bit of Jackie O. And Caitlin has the perfect body for the era. She’s tall and slender, which is great because everything from the '60s is basically a piece of fabric with two darts. It looks like nothing on the hanger, but you put it on a body with the right undergarments and it is unbelievably sexy. This shift dress (above) was modeled off of a André Courrèges design. We added a gold belt on it and Jackie O-inspired sunglasses.

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Virginia's daughter Tessa (Isabelle Fuhrman) is a teenager now. How did you approach her costumes?

During that time, some of the young girls were starting to wearing shorter skirts with tights. With Tessa, who is now 15-years-old, we could go in that direction. In the first episode, you will see her in a great pair of vintage jean shorts with a side zipper. They were a little small, but they were so fantastic that we had them stretched. We were playing off of the 1962 film Lolita—a little tomboy, but still sexy.

How far in advance do you start working on the costumes for each episode?

We have an eight-day turnaround from the beginning to the end of an episode. I spend one day doing pure concept with the script, then I start lining up pieces from our closet and pulling images to make mood boards. I build an entire book, detailing each character, before I share anything with the director and the actors. I'm a builder, so I love to sketch pieces, pick the fabric, and see it all come together.

Where else do you draw your inspiration from?

Everything! Catalogs, magazines, and even home movies and pictures. My brothers were growing up in the ‘60s, so it is fun to look back. I also have a research library with fashion history about everything from the little black dress to '60s pop. Also, since Masters and Johnson were real people, we have some of their actual clothes to draw from. The real Virginia Johnson was a little more conservative, but she certainly had sexuality underneath all of those lab coats.

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Lingerie is obviously an important part of the costumes for this show. How do you source those pieces?

The ones that you see on-screen are vintage and we have a big closet of pieces to choose from. We start every female fitting with a bra and panty girdle. It's important that we dress them from the inside out. If you don’t have the right bra, the vintage clothes will not fit properly because the bust line is higher and it’s pointier.

What are some of the other challenges you've faced with this time period?

There are logistics problems sometimes with the sex scenes. Garments need to have a front button and most pieces from this era were buttoned in the back or zipped on the side. It’s always a challenge to find something interesting for a love-making scene, so they can go from dressed to undressed without saying, “Will you get my zipper, honey?”