There are many things about Christina Ricci that seem to contradict her 30 years of age. Her physicality, for one: a just-over-five-foot frame, slipped into a sirenlike Alberta Ferretti frock, suggests a young girl playing dress up. For another, her made-in-Japan, doll-like features to match her upward lilting speech. And then there’s her self-described, less-than-adult behavior.

“My maturity level has always been extremely low for anybody my age,” says Ricci, sipping a big glass of Diet Coke and flashing a tattoo of an Edward Gorey creature on the inside of her right wrist. “It’s the letter ‘Y.’ When I was younger, I used to ask ‘Why?’ a lot. Even when I was 23 I was always like, ‘Why? Why? Why do I have to do that?’ That’s what I’m talking about with the immaturity.”

It’s a trait that has perhaps proved helpful in her stage debut in Donald Margulies’ hit play “Time Stands Still,” which reopened last week at the Cort Theatre. Ricci is the only new member in a cast that includes Laura Linney, Brian d’Arcy James and Eric Bogosian (she succeeds Alicia Silverstone, who was in the original production last winter). And as Mandy, a bubbly young girlfriend to Richard (Bogosian), whose ex is recently wounded war photographer Sarah (Linney), Ricci exudes a youthful optimism and naïveté that has her older stage fellows immediately on guard.

“She’s young, but she has the best intentions, and she’s not stupid. Her set of values is very different than [Sarah’s], but she’s there and she’s eager to learn and to be liked,” says Ricci, who identifies with not only Mandy’s values, but also her seeming inability to keep her foot out of her mouth among her more mature company. “People have been saying, ‘Oh it’s so different from who you are.’ And I’m like, ‘Well, it’s not really.’ I would totally tell somebody with scars to not worry about it because they could just get laser surgery.…And the person would probably snap at me and I would be like, ‘Oh f–k, I did it again.’”

Yet it takes a certain amount of self-assurance and sophistication to tackle Mandy, considering her co-stars and her lack of theater experience (“I’m really proud of myself for not having a nervous breakdown,” says the actress).

“She told me the very first day that she was not very smart. I don’t know what she was saying because there’s no evidence of that in her performances,” says Bogosian. “Clearly she’s very smart and also very hard working. She has a very clear method on approaching the difficult task of the ups and downs of live stage work.”

Director Daniel Sullivan was also won over by Ricci’s keenness.

“What most impressed me about her was her innate intelligence,” says Sullivan. “Joining a company that has already formed a tight ensemble would be intimidating for the most seasoned stage actor. But Christina stepped right in like she was born for the stage.”

Though “Time Stands Still” marks her theatrical debut, Ricci’s career actually began with a school play when she was growing up in Montclair, N.J. An agent spotted her and suggested she audition for “commercials and stuff.” At first Ricci’s mom, a former model, was reluctant — her three other, older children had all been similarly approached and she had refused to let them follow through — but Ricci’s siblings convinced her.

“You’re a kid, but you still know what you’re doing is supercheesy,” says Ricci of the many commercials she did before landing her first film, “Mermaids,” at age 9. “And then you get older and…I don’t know, I think your career just sort of forms.”

In Ricci’s case that career has spanned several genres, from dark and moody (“The Addams Family,” “Sleepy Hollow”) to quirky (“The Opposite of Sex”) to provocative (“Monster,” “Black Snake Moan,” in which she spent most of the film half-naked and chained to a radiator).

Asked what’s kept her going, she is relatively succinct.

“I don’t know. I just liked it,” says the actress, whose next project will be “Bel Ami” opposite Robert Pattinson. “I believe in fate and stuff. So I believe that it was meant to be.”

But that doesn’t mean she thinks it’s forever.

“I don’t want to work when I’m older. [I was] talking about somebody older who looked really good on the ‘Today’ show and I was like, ‘Man, when I’m that age, I don’t want to worry about how I look on the “Today” show,’” she says. “I just want to have a full life. And for me, I don’t think that’s going to be worrying about how I look at 70. I just want to be able to be a human and relax and, like, have some grandkids and dogs.”