Cherry Jones is either a disarming, self-effacing woman with an easy smile and a warm, down-home charm, or she's a dark and fearsome dreadnought, a monolith of cold, congealed, righteous wrath.

The difference lies in whether she's herself, the 50-year-old two-time Tony-winning actress from Paris, Tenn., or Sister Aloysius, principal of a Bronx Catholic school in 1964 in John Patrick Shanley's Pulitzer-winning play, Doubt, which begins an eight-performance run at the Merriam Theater tonight.

Jones' transformation into Sister Aloysius is one of the great feats of stagecraft of recent seasons. Critics and colleagues compare this actress, heralded by theater buffs but relatively unknown beyond that circle, to such great ladies of the American theater as Helen Hayes.

In a recent interview, Jones talked about becoming the character.

"It's not with makeup. I put on makeup after the play," she said, "a little eyeliner and some lipstick. My Aunt Carmen used to say, 'Feel a little dead, put on a little red.' "

Onstage, only an oval of face and a glint of spectacles show above a black capelet - which usually hides her hands - and below a bonnet.

"Look, I'll show you something," she said, lifting a lamp to shine more light on her neck. "That's a permanent line left there from the - I almost said the f-word - ribbon of the bonnet that I tie so tightly. That's to really pop the double chins. It also becomes sort of a ledge for me to peer down from."

Jones envisions Sister Aloysius as being a dozen or so years older than she. The nun's glasses are bifocals, with old-fashioned half-moon insets made to Jones' prescription, forcing a certain stiffness in the way she looks at Doubt's three other actors.

"They're held on by a homemade athletic band so that they will never slip down on my nose," she said. "The point is to never touch those glasses. I want her palette to be as spare as possible; she's more severe, more the monolith, that way."

The play mentions that Sister Aloysius came to the order after she was widowed during wartime, but there is no mention of details - her osteoporosis, her grating voice - that live only in the fullness of Jones' imagination.

On its surface, Doubt is about the nun's rock-hard certainty that Father Flynn, a teacher, is taking sexual liberties with a new boy in the school. She has nothing that would pass for proof, but she knows the students, knows the world beyond the church, and knows that she will fight to the death - against one of the most dominant male hierarchies of the time - to protect those children. In modern idiom, she will take him down.

For many audiences, that's enough. People leave the theater convinced either that Flynn is guilty or that Sister Aloysius is bent on destroying an innocent man.

"And both camps," Jones said, "are blown away when they find out that others don't see it the way they do. They think it's so obvious.

"I've found there are two demographics that I absolutely know how they will react. Older Jewish people will say, 'He's innocent, and she's Joe McCarthy, and it's a witch hunt.' And mothers of young children will say, 'No matter what, she did the right thing.' For those mothers, protecting children is all that matters."

Long before Sister Aloysius, there was Cherry Jones, growing up Methodist in Paris, pop. 10,000, in western Tennessee. Her father was a florist, her mother taught first grade. When she was 8, they took her to a production of The Country Girl. At the end of the play, the lights went out, but for just a second a tiny spotlight hit the actress who starred in the show.

Jones still remembers: "It was as though her soul was glowing in the dark. I thought, 'I want to do that.' "

In the late '60s, Jones lost her faith and never really found it again. She envies those who are guided by strong belief, but says, "I just don't know, and I don't know how anyone can."

She also realized that there was something about her that would not play well in that place and time: She was gay.

Jones has never kept that a secret in her work life. She mentioned her then-partner, architect Mary O'Connor, when she accepted her first Tony in 1995, and her current partner, Sarah Paulson (Harriet on Studio 60 on the Sunset Strip), when she won again in 2005. But for quite a while there was an unspoken agreement not to talk about it back home.

She said that more recently her mother says she has become all right with it intellectually, if not so much emotionally. As for Paris, Tenn., "There was a New York Times piece on me a while back, and after that came out there were lots of letters to the local paper citing Leviticus, but that's changed. I go back a couple of times each year. I was grand marshal for the parade in the major event of the town, the annual World's Biggest Fish Fry."

Before she returns home again, though, there's a bit more Sister Aloysius to be, closing out Doubt's national tour in Philadelphia.

"I find myself defending her all the time. She's not just this block of granite," Jones said. "Catholics tend to love this play, since it's a fair and loving depiction of the church. Nuns and priests love it. Kids see her as a protector, and they love that she is so fair and so just and not arbitrary.

"If I were in the audience, I would walk away with doubt, but I know the play from deeply inside her. I see the noble, serving part of the woman. On nights when I sense the audience just hates her guts, I'll tweak the reading of the end of a line a little to open a door to her heart just a crack.

"At one performance, at the very end, just as the lights went off, a woman in the audience said, 'May she burn in hell.' Well, I'll tell you something about Sister Aloysius. I adore her."

Theater

Doubt

Tonight through Sunday at the Merriam Theater, 250 S. Broad St. Tickets: $25-$64.50. Information: 215-732-5446, www.BroadwayAcrossAmerica.com