Amanda Abbington, Martin Freeman, Benedict Cumberbatch

You are cordially invited to share in the celebration as John Watson and Mary Morston are united in marriage. As long as you meet the best man's standards, that is.

On Sunday's all-new Sherlock (check local PBS listings for times), the quirky private investigator decides to take his duties as best man very seriously. Some would say a little too seriously. "The wedding is designed by Sherlock. He has a big say in what goes on," Amanda Abbington, who plays the blushing bride Mary, tells TVGuide.com. "He's sort of their wedding planner, which is very sweet. He's a high-functioning sociopath so he has to know what's going on all the time."

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Viewers first met Mary in the season premiere, when she was present during the reunion between Sherlock (Benedict Cumberbatch) and her fiance John (Martin Freeman). In real life, Abbington and Freeman are longtime partners who have two children and have worked on a handful of other projects together before Sherlock.

Creators Mark Gatiss and Steven Moffat had always asked if Abbington was interested in a part, but gave no clue what it would be. When they sat down with Freeman to discuss the third season, they invited Abbington to join them. "Then they gave us the broad outline and said, 'We're going to introduce a wife for John,'" she says. "I thought they were going to say, 'Do you know any good actresses that might work well with him?' Then they said, 'We would like you to play Mary.' I was just flabbergasted. I think she's such an amazing character. I love Mary Morston, and to play her is a real honor. So, I was thrilled. Of course I jumped at the chance.

"There is slight nepotism there, of course, because I've been with Martin for 13-1/2 years," Abbington adds. "We have a shorthand. We've worked together before. We're very easy with each other. I think Mark and Steven knew that. They wanted something that wouldn't cause friction. I think I slotted in rather nicely."

Mary not only gets along with Sherlock, but surprisingly takes a liking to the offbeat detective. "Mary has fallen completely in love with John. She's had her head turned by him," Abbington says. "But she's very comfortable in her own skin. So when you meet her, she's not intimidated by Sherlock and she's not intimidated by Sherlock and John's potential relationship."

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That confidence pays off when she has to smooth things over between the often combative partners, who are uncertain how their friendship my be affected by the looming marriage. "She does everything she can to bring them together," she explains. "As she says in her first episode, she likes Sherlock. She thinks he's interesting and... she sees a lot of herself in Sherlock and a lot of herself in John."

During the recent Television Critics Association winter previews, Cumberbatch said about Sherlock and Mary, "I think there's a synchronicity... You get to understand why there is that immediate connection... as the episodes unfold in the series. I don't feel that it's anything other than a partnership, another version of the John and Sherlock partnership. It tempers certain variance of the emotional clashes between the human John and the machine Sherlock, if you will. And, you know, it's within that balance between both of them, she's loved by both of them. There's a huge amount of respect both ways."

Although it's never explained how Mary and John first met, Abbington and Freeman have their own theory. "We think that because he's a doctor and he works at the surgery, we think that she's done her research and found an opening to start her new life," she says. "She's applied for a job as a receptionist/nurse at your local doctor's surgery. It's just coincidental that he works there. I think she's like a trained nurse. She wears many hats. She can adapt and blend in to where she needs to be."

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As an actress, Abbington also wears many hats. After Sherlock concludes, she'll appear in Masterpiece's Mr. Selfridge, which enters its second season in March. Abbington reprises her role as Miss Mardle in the period drama about Selfridge & Co., the first department store to use modern merchandising techniques thanks to its founder, Mr. Harry Selfridge (Jeremy Piven).

When last we left Miss Mardle, she was left heartbroken by Selfridges' chief of staff Mr. Grove (Tom Goodman-Hill) who had strung her along and the jilted her for a younger woman. "Mr. Grove is still horrible to her. He's vile to her in this series," Abbington says. Unfortunately, poor Miss Mardle's anguish doesn't end there. At the beginning of the season, her brother will also fall ill and die.

Not all is lost, however. "He passes away, but she comes back with a huge house that he's left her and a disposable income," Abbington says. "She becomes a woman of independent means, but she still loves her job at Selfridges, so she still works there. With that money and that security comes a bit of confidence, and she starts to believe that actually she's not a bad person, that people might like her. That turns a few male heads. A young Belgian refugee comes to stay and falls in love with her, but it's whether she follows her heart or her head."

Shooting Mr. Selfridge and Sherlock concurrently took no small amount of planning. "It did mean a lot of running around the country for about three to six weeks," Abbington says. "I did have a piece of paper in my kitchen. It was like a military operation of where I was going to be and what time I had to be [there]. And then at one point, I had to bring my children with me because they were on summer holiday. So, I was dragging two little dudes with me as well. It was great fun, though."

Sherlock airs after Downton Abbey on PBS' Masterpiece on Sundays. Mr. Selfridge will kick off its second season on Sunday, March 30. Check local listings for times.