"I had a natural antipathy towards her, for any number of reasons," says Mulgrew, who's best known for her portrayal of Kathryn Janeway, the first female starship commander, on TV's "Star Trek: Voyager."
Written specifically for Mulgrew by Matthew Lombardo, "Tea at Five" begins a three-month run at the Cuillo Theatre in West Palm Beach Saturday.
In an interview from her New York apartment, Mulgrew says she had been compared -- not always favorably -- with the great Kate over the years. She says she found the legendary actress "strident."
"But I fell in love with her during the rehearsal process, and in the playing, I really did. If you've very lucky in this craft, you find the hook, the ingredient that makes the recipe perfect for you.
"And in this case for me it was her vulnerability -- which is, for lack of a better word, sublime. It's the cover-up that has made her such a compelling personality, because next to that maverick toughness is a fragility as ineffable and poignant as an October afternoon."
Mulgrew, 48, has been doing the show for nearly two years off-Broadway. She says she's read every word that's ever been written about (or by) Hepburn, and has seen all of Hepburn's films "at least 10 times. And 'The Philadelphia Story,' 50 times.
"I've mused, reflected, walked," Mulgrew says. "I've gotten myself in shape physically. I had to learn how to jump three octaves vocally, from the first act to the second act.
"But finally, having said all that, you throw it all away, and either the marriage takes place or it doesn't. Very happily, in this case, it did.
"The job of the actor, as opposed to the impersonator, is to realize the character. So if you come to the theater and you say to your wife, 'My God, she's just doing an impersonation,' I've not done my job at all."
In Act One, Hepburn is 31 years old and has just been branded box office poison. "We watch her agitation," Mulgrew says. "We see what it was that made her the sleek and powerful young animal that she was. And underneath that too, what it is that makes the tiger pace so unrelentingly."
Hepburn is 76 in the second act, "reflecting and self-deprecating, the Hepburn we all know and love. The Hepburn of 'On Golden Pond.'"
Mulgrew, who first rose to fame on the soap opera "Ryan's Hope," is still recognized the world over for her performance in the "Star Trek" series, which ended in 2001.
"Did I have some trepidation about taking that?" she says. "Yes. But very small, because it was a wonderful role. I knew that it was going to be an extraordinary chapter in my life, which it was, and if I'm typecast because of it that's probably my problem and nobody else's."
For ticket information and more, call the Cuillo box office at 561-835-9226.