Natascia Diaz as Fosca and Claybourne Elder as Giorgio in “Passion,” the Stephen Sondheim musical at Signature Theatre though Sept. 23. (Margot Schulman)

How do you convince an audience that the female lead is unattractive to all the male characters at the beginning of the show yet irresistible to the leading man by the end? That’s the challenge of reviving the 1994 musical “Passion” by songwriter Stephen Sondheim and playwright James Lapine, as Arlington’s Signature Theatre is doing this month.

The challenge is even greater when Fosca — the homely, sickly woman romantically pursuing a handsome Italian army officer who has a beautiful mistress — is played by an actress as lovely as Natascia Diaz, who returns to Signature after memorable turns in “West Side Story” and “Threepenny Opera.” The first decision, says director Matthew Gardiner , was to rely on acting rather than makeup.

“There’s a temptation to make Fosca a witch with warts,” says Gardiner. “But a lot of what Giorgio and the soldiers find distasteful is her behavior. She’s a woman who has experienced an unkind life, one who has been told that she’s dying. She decides she’s going to live the rest of her life the way she wants to, and that’s what’s off-putting to the men.”

“I don’t see any need to make any physical alterations, such as a hunched shoulder or big nose,” adds Diaz. “A lot of the time, ugly is all about how you feel. As an actress, you don’t play ugly; you play frightened. You play dismissed and isolated. You play the agony of having all this tenderness and insight that you can’t share.”

This story was first told as Iginio Ugo Tarchetti’s 1869 novel “Fosca” and then as Ettore Scola’s 1981 movie, “Passione d’Amore.” Tarchetti enjoyed the roominess of a novel to thoroughly explore Fosca’s backstory and inner thoughts. The first thing Diaz did after being cast in the role was to read the novel.

“It was a gut punch, to say the least,” Diaz says. “Everything is there: her delicacy, her sweetness, her pure need, her self-knowledge. She says, ‘The only thing worse than my hideousness is my sensitivity.’ ”

Claybourne Elder as Giorgio and Steffanie Leigh as Clara. (Margot Schulman)

For all the aspects of the book that had to be left out of the libretto, Sondheim compensates with melody. As the show begins, Clara the mistress has the buoyant, major-key tunes, while Fosca has the brooding, minor-key numbers. But as Giorgio’s affection gradually shifts from one woman to the other, the music changes as well, with Fosca taking over the hopeful, bright passages and Clara left with the despairing dark parts.

“You know how the character is feeling about an idea or emotion from the melody,” Diaz says. “It gives me chills. Before you know it, Clara’s singing something that Fosca sang a half-hour earlier, so you know she’s feeling what Fosca felt earlier. And at the end of the show, Fosca is singing like Clara did at the beginning. The audience says, ‘Ah, that’s now her song; she’s taken on Clara’s happiness.’ ”

Gardiner suggests that the key scene occurs when Giorgio goes to the station to take a train back to Clara. Giorgio, played by Claybourne Elder, is upset when Fosca, resembling a stalker, pops up on the platform to declare her love for him. “Your appearance is no excuse for the way you behave,” he tells her. “My feelings for you are a result of your relentlessness.” “I’m sorry,” she responds. “No one has ever taught me how to love. I know I feel too much.”

“That scene,” Gardiner says, “tells us that this is not the story of Giorgio coming up to Fosca’s level. It’s the story of the two of them teaching each other how to love. Giorgio tempers Fosca, and she opens him up. I’ve seen productions where Giorgio is constantly pushing her away, and you wonder at the end how we got there. You have to drop seeds along the way that Giorgio is drawn to her even as he’s resisting her. That’s hard to convey, but Natascia and Clay are doing a good job of balancing that.”

If you go

Signature Theatre, 4200 Campbell Ave., Arlington. 703-820-9771.

Dates: Through Sept. 23.

Tickets: $40-$104.