Shepherd played Stewart in NBC's top-rated "Martha, Inc." in 2003 and has reprised the role for "Martha: Behind Bars," airing this fall on CBS to dovetail with the launch of Stewart's new syndicated show.
"I looked at it as a completely different project and a completely different character," Shepherd says of her second go-round as Stewart when she called Wednesday to discuss the upcoming release of her classic series, "Moonlighting," (search) on DVD.
"I didn't even refer to the prior movie, and I haven't watched it since it originally came out," she says. "I thought I'd watch [the first movie], but then I realized, no, this is a different movie and a different person.
"It's a great opportunity for me to play such an extraordinary woman who's had such a profound effect on all of us."
"Martha: Behind Bars" will document Stewart's very public downfall, culminating in her prison sentence for lying to federal investigators and her subsequent post-prison renaissance.
"It's a fascinating, ongoing story, one of the great stories of all time, I think," Shepherd says of Stewart's life. "She's a great woman, and I have great admiration for her, now more than ever.
"She's totally being treated unfairly. It was very personal over a very small mistake and she's suffered, she's paid her dues and she came out on top."
As has Shepherd, who looks back fondly on "Moonlighting," the stylistically groundbreaking ABC series (1985-89) on which she starred with Bruce Willis — she as beautiful ex-model Maddie Hayes and he as wisecracking private eye David Addison.
The first two seasons of "Moonlighting" arrive on DVD May 31 (Lion's Gate), with Shepherd, Willis and series creator Glenn Gordon Caron providing commentary.
"I always say it's really hard to make a bad movie, and it's really hard to make a good one," Shepherd says.
"It can't be a great project unless it has some kind of magic that stirs the pot — and I believe that was 'Moonlighting,' with all of our talents in a pot, over a fire, bubbling.
"It was a magic cauldron that we cooked up — it all had to work right, and it did," she says, before answering the inevitable question about a possible "Moonlighting" reunion.
"There have been many talks about it ... and I've always said I'd love to do it ... and I think Bruce would do it," she says.
"After I did the [DVD] commentary with Glenn, I asked him [about the reunion movie] and he said, 'I just don't know how to do it. He had this pained look on his face.
"This show was so great, we did this wonderful thing; [a reunion] would be hugely watched and probably horribly disappointing.
"But if Glenn could figure out a way to do it, I'd do it."