Cherry Jones remembers the exact moment she decided on an acting career. Raised in Paris, Tenn., she enrolled in a theater workshop at Northwestern University when she was 16. That summer, she saw, in its pre-Broadway stage, the now-legendary Colleen Dewhurst/Jason Robards production of O'Neill's A Moon for the Misbegotten.

She still remembers Dewhurst's extraordinary presence: "Powerful … and at the same time so vulnerable. … I'd just never seen an actress like that before."

Jones also remembers what she asked Dewhurst in the post-show Q&A: "'How do you keep it fresh night after night?' [Dewhurst] threw her head back and laughed that unbelievable husky laugh of hers, and I remember her saying, 'Ya don't. You learn to fake it.'"

Thankfully for audiences, that's one piece of advice that the 50-year-old Jones has never taken to heart. An incandescent performer whose immersion in her characters is so total that she's hardly recognizable from one role to the next, she comes to Tampa this week in the play for which she won the second of her two Tony awards for best actress: John Patrick Shanley's riveting Doubt.

As Sister Aloysius, the head of a Catholic elementary school in the Bronx c. 1964, Jones plays a doctrinaire disciplinarian who becomes convinced that a charming young priest at the school is sexually abusing a young African-American male student.

"I remember thinking that because of her certainty this is a part that could grow old very quickly," she reflects, speaking by phone last week from an Acela train to New Haven, Conn., where Doubt was about to open. "I've done her now over 650 times. I feel so blessed at having had so many cracks at her."

Jones' warm and gracious demeanor is about as far from the dour Aloysius as one could imagine — and yet she is utterly convincing in the role. And the volcanic absolutist Aloysius is in turn a far cry from Catherine Sloper, the shy spinster in the title role of The Heiress, for which Jones won her first Tony in 1995. The many women Jones has played in numerous small but indelible roles in films like Erin Brockovich and Ocean's Twelve all seem to occupy their own individual universes, too.

It's versatility like this that has led to Jones' reputation as one of the best actresses — perhaps the best stage actress — in the United States. And then there's the simple fact that she does her homework, making specific decisions about her character's past history and inner life. For instance, there's a turning point in Doubt "that just has got to crack Aloysius in half," says Jones. Even though the playwright leaves ambiguous what might be going through Aloysius' head at that moment, Jones has to be certain.

"It matters very much to me, the actress, that I make it as clear as I possibly can what I think that moment is," she says.

Jones didn't think at first that she could master Aloysius' harsher side. She credits director Doug Hughes with leading her there.

"He helped me make her much more rigid," she says. "I'm Methodist from Tennessee, and I just didn't quite understand — I'd heard all the stories about nuns but hadn't experienced them."

Once she finishes her tour in Doubt, which invariably leads audiences into post-show debates about guilt and innocence, Jones is looking forward to returning home to NYC and fulfilling a somewhat ironic obligation: jury duty.

"Isn't that funny? I've been working pretty solidly now for the last two years, so I keep missing [jury duty]," she says. "I have to go down and apologize. I always want to serve and I always get picked. Everyone looks at me and sees someone they think they can influence because I can easily put myself into other people's shoes."

Even when those shoes are the clunky brogans of Sister Aloysius.

Doubt, Tampa Bay Performing Arts Center, April 24-29, 813-229-STAR.