"I think in some way she looks younger this season than she ever," says actor Caitlin FitzGerald, who plays her, "Which I think is kind of interesting given where she is going." Though the gradual change of wardrobe has helped.
"I have to admit so much of the work is done with the clothes. Especially with the 50s clothes - we wore very restrictive undergarments underneath the clothes and so you just had to kind of stand in a very 1950s kind of way that is really different to me in my ripped jeans." Masters of Sex may be the story of the relationship between Masters (Michael Sheen) and Johnson (Lizzy Caplan) and their pioneering research.
But the character of Libby has been increasingly pivotal to the drama loosely based on the 2009 biography Masters of Sex: The Life and Times of William Masters and Virginia Johnson, the Couple Who Taught America How to Love by Thomas Maier.
"There is not a ton in the book about Libby, so of all the characters we have taken the most liberty with her. And it's been great because in the book she has been written as bit of a doormat unfortunately. We were all not interested in that. We have gone to all types of places with her."
FitzGerald says she was initially reluctant to sign on to the series because she wasn't interested in having to play a typical timid 1950s housewife. But the show's creator, Michelle Ashford, assured Libby would become more interesting as the show went on "and they certainly made good on their promise. She starts out the most naive and innocent of all the characters and then by the end of this season is seeing the most clearly.
"She was a girl in a lot of ways and she called Masters 'Daddy' - I don't know if you remember that particular piece of weirdness - and she has been growing up. And with that comes a loss of innocence."
With her blond hair, the period and her marital circumstances, she might remind some of a certain other television character.
"I was terrified that all anyone would say is this is a poor man's Betty Draper. They both have philandering husbands. But I think where Betty Draper gets really hard, Libby is a really emotional person and has this big heart. That is how I see them as divergent." Like Betty in Mad Men, she's put up with a lot already and it looks like there is more to come.
"She sure does," laughs FitzGerald.
"Every time we start shooting a series I am so excited to get back to work and a couple of weeks in I'm 'man, I feel really depressed why am I so blue' and I realise it's from playing a character who is routinely ignored and dejected. It takes a toll." The third series begins with Libby seemingly accepting the Masters-Johnson relationship and that she's now part of a three-way marriage.
The first episode has Libby and Virginia sharing a kiss, which seems to seal the deal.
"It does and it doesn't. Libby and Virginia have always had a very strange fraught relationship. In a lot of ways Virginia is one of Libby's only friends which is a strange dynamic with the woman you know is sleeping with your husband.
"So there is a 'keep your friends close and your enemies even closer' mentality to the whole thing. And that moment in the first episode - the kiss - is wanting to know what is so special about this creature that Bill is so obsessed with. And kind of a moment of gratitude - that Virginia is understanding what I am asking her and that we are making this pact.
"I think it is all those things before it is a sexual moment."
Outside the show which takes five months out of her year, FitzGerald is balancing commitments to film roles - she played Meryl Streep's daughter in comedy It's Complicated - and has occasional forays on the New York stage.
Yes, she has had offers for other Libby-like characters but: "I have been really careful. I need go play a junkie or something."
What: Masters of Sex series three
When and where: SoHo, Sunday 7.30pm
- TimeOut
