After ‘L.A. Law,’ Life Goes On, Onstage : Theater: Alan Rosenberg reconnects with the theater--and with his departed brother--in the acclaimed ‘Old Business.’

His endearing performance as the sensitive, soulful attorney Eli Levinson on ABC’s “Civil Wars” and later on NBC’s “L.A. Law” made Alan Rosenberg a hot TV star and a “thinking woman’s” sex symbol.

Rosenberg and co-star Debi Mazur, who played Eli’s secretary Denise, proved so popular with audiences that after “Civil Wars” was canceled in the spring of 1993, they joined the cast of “L.A. Law” last fall. They also created TV history: They became the first actors from a prime-time series to join another series on a different network as the same characters.

But when “L.A. Law” closed up shop in May, Rosenberg didn’t rush into another series or seek out a TV movie or feature. Instead, he opted to return to his theatrical roots.

Rosenberg, 42, stars with Harold Gould in Joe Cacaci’s critically acclaimed drama “Old Business,” which finished a sold-out engagement at the Court Theatre earlier this month and reopened for a six-week run Thursday at the Coast Playhouse.

Rosenberg was hoping to direct his Emmy Award-winning wife, Marg Helgenberger (“China Beach”), in a play this summer when “Old Business” came his way. “I love doing theater,” said the fast-talking Rosenberg, as he relaxed on a bench in the Court Theatre’s courtyard after a recent performance.

“Until I did ‘Lost in Yonkers’ on Broadway three years ago, I hadn’t done a play in eight years. I want to make sure I don’t stay away from it for that long (again). You get so sidetracked--you want to do something that will further your career and make you some money. What I originally fell in love with about this profession was coming into contact with people. It’s truly gratifying.”

Though the actors never share the stage together, “Old Business” focuses on the emotionally charged relationship between Abe (Gould), a 76-year-old real-estate tycoon being forced into retirement for health reasons, and his eldest son David (Rosenberg), to whom Abe reluctantly hands over the business.

David must struggle to gain his father’s respect, act as guardian to his younger, alcoholic brother Edward and keep control of his father’s business.

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Rosenberg said that his involvement in “Old Business” was pure happenstance. He was doing a play reading at the Westwood Playhouse earlier this year when he caught the eye of Cacaci, who, along with “Old Business” producer Dan Lauria, operates readings at the Playhouse every Monday evening.

“They asked me to do a reading (of “Old Business”) at the Westwood Playhouse,” he recalled. “Joe Cacaci had written it for a friend of his in New York, but he saw something in me that said to him that I could do it. I was flattered he thought I could do it.”

And Rosenberg thought it would be good to be seen in a play “where I was on stage for a length of time. It just all came together. Then we got all roped into producing it together,” he said, laughing. “I wasn’t necessarily looking for something to throw my own money into, now that I am out of work. But it’s worked out well.”

Cacaci, who also is the director, said Rosenberg has the ability to effortlessly juggle both the comedic and tragic aspects of the piece. “He does that just so seamlessly,” Cacaci explained. “That’s really the key to the play, to be able to do that. Those are layers and details that are very difficult to do and he had them the first day of the reading. He understands that part in his heart.”

In the character of David, Rosenberg sees a lot of his late brother, film producer Mark Rosenberg (“The Killing Fields”). Mark Rosenberg died of a heart attack in November, 1992, at age 44.

“My mother was crying (at the play),” Rosenberg said softly. “I sort of played Edward to my brother’s David all of our lives. He was not as frazzled as my character in this, but he did take care of me. I was sort of getting into trouble and always knew that my brother would be there backing me up, which is disconcerting now since he’s not here anymore.”

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Still, Rosenberg added, “I know the difference between fact and fiction. I’m able to use whatever is bothering me in my work. It’s not daunting. It’s cathartic.”

The Passaic, N.J., native earned a bachelor’s degree in political science and theater from Cleveland’s Case Western Reserve University. He applied to only one graduate drama school, the Yale School of Drama. His drama professor didn’t think he was ready to audition for such a prestigious school, but, Rosenberg said proudly, “I got in.”

Rosenberg paused. “It’s funny,” he reflected. “The older I get, the more I look back at my life. I kind of find we all get on a path. No matter what you do to sabotage it, no matter what other people do, you keep going on that path. I had no idea I was going to be an actor.”

Dropping out of Yale during his second year, Rosenberg went to New York. “As soon as I went there I did plays for no money but I was driving a cab, a limo, all kinds of odd jobs. It took me three years before I became somewhat known.”

And he became “somewhat known” in 1976, winning rave reviews for his role as a vulnerable drug addict accused of murder in Thomas Babe’s “A Prayer for My Daughter” at Joseph Papp’s Public Theatre.

For 10 years, Rosenberg said, “I worked constantly in the theater. It was fine. But I wanted to do other things.” So he came out to Hollywood a decade ago. “I thought I would test the waters out here as far as acting is concerned.” Until he landed the role of Eli in 1990, Rosenberg primarily did guest spots on episodic TV.

As for the future, Rosenberg just wants to “keep on working. It’s been a great job for me on ‘L.A. Law’ and ‘Civil Wars.’ Actors are not supposed to be employed for more than three years, so I wasn’t devastated (both series were canceled). But I did enjoy it. I had a ball playing this character who seems to be so lovable. People who come to see this play love Eli . . . so I am going to look for something like that.”

* “Old Business” runs Thursdays-Saturdays at 8 p.m.; Sundays at 3 p.m. through Aug. 21 at the Coast Playhouse, 8325 Santa Monica Blvd., (213) 466-1767. Tickets $22-$25.