"It's that fine, fine, fine line between reality and just being a actor," the 52-year-old actress recently told reporters on the show's New York set.
Referring to the cast and crew as "the best of the best," she confessed she felt like she had "to bring it" when she showed up for work.
"You have got to raise your game and that's what you want as an actor," Shields said, adding she has "SVU" star and producer Mariska Hargitay to thank for the role.
"I feel lucky. Mariska is single-handedly responsible for it," Shields recalled of how Hargitay approached her.
"She saw a guest spot that I had done for Ali Wentworth's show 'Nightcap' and it was a sort of disturbed -- more comedic -- but a very disturbed, kind of multi-faceted character. And [Hargitay] said on that day, 'We've got this story line and not only do I want to direct you, but there is so much more to you than we've had a chance to see.'"
Admitting she has been locked for years "in this 'Suddenly Susan' world or this more buoyant type of [role] and loving it," Shields said she was ready for something with more grit when Hargitay approached her about tackling on-screen serious issues such as how we determine what is in the best interest of a child.
"That really poses an interesting predicament for both of us," Shields said of her character and Hargitay's Olivia Benson, who is a police lieutenant and single mom to an adopted son named Noah.
"We're both fighting each other and, yet, we are both fighting for the same thing, but we can't make it about one winning."
Shields said she has long been an "SVU" fan, but hesitates to view episodes in their entirety with her daughters.
"Now I have younger kids and they want to watch it, but sometimes you're like, 'Oh, god, you can watch this part, but not that part.' It's part of everybody's family, just because of the topics it is based on. It really delves into things that we need to face. I appreciate the show. I love it," Shields said.
The actress noted her guest appearance on "SVU" is a bit of a homecoming since the series airs on NBC, the longtime network of her former sitcom, "Suddenly Susan."
"They let you out, but you're not allowed to stay out. They bring you back in," she laughed.
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