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In the show, Dr William Masters has tremendous passion for his work but he lacks passion for his own wife.

MA: Dr William Masters is a very complicated character. He was very, very passionate about his work but we see in the pilot that he comes home and is less passionate with his wife. I think it's no mystery that he was drawn to the world of sexual study because he himself had some demons he was trying to sort out.
He came from an incredibly abusive background. He had a tyrannical father and he was a man who had had to put on a real suit of armour to just get up every day from the time when he was fifteen on. But underneath that suit of armour are a lot of gaping holes. What you will see in this character is many contradictions, many different facets of his personality.
I think that the way sometimes people who have emotional problems end up being shrinks, the fact that Dr William Masters ended up being a sex researcher makes perfect sense.

We've talked about D William Masters. Let's talk about Virginia Johnson...

MA: Virginia Johnson was 31 when she met Dr William Masters. Twice divorced with two kids, she had been a lounge singer for her previous husband, a bandleader. She was a little bit adrift. Virginia was not a college graduate but she had a burning sense of ambition, even though she wasn't sure where it should be channeled.
She applied to the University of Washington as a secretary, with the idea that she would take night classes and eventually make something of herself. But it was quite vague as to in what area she would make her mark. She started working for Dr William Masters when he was just launching his study. And, as a prostitute had told him, if you really want to know about sex you're going to need a female partner. This was a very happy coincidence, of finding kind of the 'perfect woman' to help him with his work.
She was also ahead of her time in many ways. Especially when it came to the subject of sex. She was not at all prudish or squeamish or embarrassed. She had a very active sex life. Masters' wife Libby was a very traditional woman. They were very different and yet they ended up becoming, oddly enough, very good friends, despite the fact that there was, in fact, a relationship going on with Virginia and Bill. sources