What a feline!

In the award-winning musical Cats, the show-stopping moment invariably comes when old cat Grizabella sings Memory. The current production at the Walnut Street Theatre features South Philly performer Katie O’Shaughnessey in the role. It’s a part she’s wanted since age 9, when she saw her first production of Cats — the longest-running musical in Broadway history.

"I remember thinking, ‘I couldn’t dance this show, but I could sing Grizabella,’" recalls the 28-year-old actor, who lives in the Girard Estate area.

Her prediction proved true. Last spring, she auditioned for the role in the Walnut production. Of course, she had to sing Memory.

"I was very nervous because I wanted the role so much," she recalls. "But the audition went really well. I sang my best."

Four days later, while O’Shaughnessey was in Shop Rite at 26th and Oregon, her cell phone rang. It was the Walnut Theatre’s managing director, offering her the coveted role.

"’Omigod, yes!’ I screamed into the phone," she describes. "I was so excited, I was dancing in the aisle."

Now her excitement is playing out onstage. Cats opened at the Walnut Theatre last night and runs until Jan. 9.

The show featuring music by Andrew Lloyd Webber endured on Broadway for l8 years. Set in a junkyard, the play is about an annual gathering of the Jellicle Cats. One of them is the aging cat Grizabella.

"I have to show what it feels like to struggle for every step," says O’Shaughnessey. "And this is not an old woman — it’s an old cat, and that has to be believable."

Grizabella has her big moment in the second act when she sings the full version of Memory. She sings a briefer version in Act I, so the nickname for the full song is Big Memory. It’s always the climactic scene.

"The big joke about Cats is that all the other cats work their butts off for the entire show. But Grizabella is onstage for only about 25 minutes, and she steals the show," says O’Shaughnessey.

It could be argued, though, that performing Memory alone requires the entire play’s worth of talent and stamina.

"It’s a very emotional, huge, belting-out song," says O’Shaughnessey.

Another challenge comes at the end of Act II. Grizabella is chosen to be reborn, so she ascends a staircase that will lead her heavenward. On the Walnut stage, the staircase is almost 60 feet high, rising above the curtain line. And as O’Shaughnessey climbs, there’s a spotlight on her face and smoke around her, making the climb even more difficult.

"I was nervous about this at first," she confesses. "I didn’t want to trip or do something really stupid, especially with so many people watching!"

But her fears were soon eased. Before the technical rehearsals began, the tech staff brought the actor on the stage to practice climbing. "I felt more comfortable than I expected," she says. "It felt very secure and safe."

In her dressing room each night, O’Shaughnessey applies the elaborate makeup needed to transform from a youthful, fresh-faced actor into an aging feline.

At first, it took a full hour, but by the time previews began last week, she’d trimmed it to 20 minutes.

"And it’s really fun to put on," she says, describing the two big stripes across her face, brown and black accents for her eyes, smeared red lipstick, small whisker dots, plus a small black outline on her nose. "If it’s too big, I look like a squirrel, not a cat."

O’Shaughnessey is back in human form during an interview in a lounge area of the Walnut Street Theatre, where the actors can relax before or after rehearsals. Despite a strenuous rehearsal that day, O’Shaughnessey exudes energy and enthusiasm. And her voice, even when she’s not singing, has an Irish lilt.

Performing in Cats for the first time — after having seen many versions — she’s delighted to work with a director and a costar who have extensive experience with the show. Ken Prymus, who plays Old Deuteronomy, performed the role on Broadway for seven years, setting a record for a featured male performer.

Director Richard Stafford was the dance supervisor for Cats on Broadway, and has directed national tours of the show. He’s directed O’Shaughnessey in other productions, including her first role as a principal equity actor.

"He’s really been a key to my career, and I adore him!" she enthuses.

O’Shaughnessey, a Detroit native, has been on the stage since age 7, when she played an orphan in a community theater production of Annie.

She went on to perform the leads in high-school musicals, including Maria in West Side Story and Eliza Doolittle in My Fair Lady. At Yale University, O’Shaughnessey was a double major in theater studies and computer science.

This unlikely combination served her well after graduation, when she headed for New York to launch her acting career. She worked full-time as a computer programmer while doing freelance acting until she could focus on theater full-time.

Moving to Philadelphia, O’Shaughnessey settled in South Philly and has been busy on local and regional stages. At the Walnut Street Theatre, she’s had roles in several mainstage musicals, including La Vie En Bleu, Hello Dolly and, most recently, Annie Warbucks. Now Cats marks another milestone in the actor’s career: her first leading role on a Philadelphia stage.

Offstage, O’Shaughnessey is also performance-driven, with some demanding athletic roles as both a marathon runner and mountain climber.

It began when her partner, Katie Kenney, encouraged her to run in a New York marathon.

"I wasn’t really convinced I could do it," O’Shaughnessey says. But it turns out she could — and she has run in four marathons so far.

They were good preparation for another impressive achievement: climbing Mount Kilimanjaro. She and Kenney both were participants in a project sponsored by the Achilles Torch Club, an international organization for disabled athletes. Kenney, who is deaf, was a member, and O’Shaughnessey was a volunteer.

In summer 2001, the New York chapter was invited to climb Mount Kilimanjaro by a member of the South African chapter. O’Shaughnessey and Kenney both decided to try. They were in a group that included seven disabled athletes and 20 able-bodied ones.

The peak in Africa soars 19,344 feet. After six-and-a-half days of climbing, all but two members of the group made it all the way to the top.

"We were totally exhausted," recalls O’Shaughnessey. "But there was also this sense of awe that we had done this. It was intense and exhilarating — absolutely a life-changing experience."

Even since she has started rehearsing for Cats, there hasn’t been much time for other pursuits.

It’s been yet another experience she can share with Kenney, who, as a theater dresser, helps the cast backstage and assisted in preparing the costumes.

In her costume as Grizabella, O’Shaughnessey lives out her childhood dream.

"The most exciting part is singing that song every night," she says. "The words are so beautiful! It’s just wonderful material to work with."

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The Walnut Street Theatre production of Cats runs until Jan. 9. Tickets are $10-$65 and are available by calling 215-574-3550 or online at www.wstonline.org.